Since the old thread is pretty outdated (and Elizara nagged me bout it ) I'll write up a new guide how to battle those nasty buggers (ad/spyware, viruses, trojans and the like) and hopefully get your system clean again.

It has been preached since the dawn of time, and I will start with saying it again:
Create a bucking backup!
With the era of crypto trojans upon us there is not much to clean/rescue when that thing already encrypted all of your valuable porn, eh, i mean important documents.

If you wanna (or have to) go cheap, get a USB stick for a few bucks and copy your most important stuff onto it once a week/month/whenever you remember it.
If you want to get it done automatically there's also enough free ways, be it backup of certain files/folders to pretty much any online or local storage with Duplicati or a complete backup/image of your machine with Veeam Agent for Windows. For up-to-date links to more programs, check the freeware list.

Another thing to take care of beforehand is a decent AV program. With Windows 10 you're already good to go, Windows Defender got better and is pretty much en par with commercial scanners by now. If you still wanna use a 3rd party scanner, stay away from those darn "internet security" packages and get just the AV scanner, no need for that added bullshit.

Also, there is one infection source that needs quite a lot of training to close: You! A lot of infections happen because of careless users that got tricked into opening shady mail attachments or opening fishy links. I know that it's easier said than done to not click that stuff, but just take those extra 10 seconds to check a few minor details. Do the sender name and e-mail address/domain match? If it's meant to come from a company, does it include all the company details and contact information commercial communication has to include? Did you even order something from that site, are you a customer of them and/or are you waiting for a delivery? And please remember your bank will never send you e-mails about security-relevant stuff like PINs or passwords.

Cleansing time!
Now for the main dish, getting rid of whatever slipped past your AV.
What to do highly depends on what infected you.
  • If it's "just" annoying stuff like browser toolbars, adware or so called PUP (potentially unwanted programs), grab Malwarebytes and/or Spybot S&D, scan your computer with them and let them remove anything suspicious they found.
  • When your system behaves different than usual, like high(er) CPU usage even though you're not running anything special, or high hard drive activity out of the blue, look into task manager what is causing the load. When in doubt, shut your system down and start with offline scans (see below).
  • When you caught a crypto virus and notice that the typical "your files are encrypted" notices pop up, pull the power cable! If you're running a laptop or can't reach the cable, hold the power button for ~5 seconds to perform a forced shutdown. Perform offline scans (see below).

Offline scans (or, to be precise, scans from live systems that boot from a CD/DVD/USB stick) are the best way to deal with infections that might have dug deep into windows and are hard to detect/remove from within the running system. Since your infected system should be turned off, you will need a second PC/laptop or ask a friend/family member to download and create a scan media for you.

Some AV companies offer free rescue/scan disk images that you can either burn to a DVD or create a bootable USB stick with. For example, Kaspersky and ESET offer such rescue systems, usually in the form of an ISO image.
Download the ISO, get the little tool Rufus and use it to write the ISO image onto a USB stick and make it bootable.
Short usage note for the current version of Rufus (3.4): select your USB stick from the "device" drop-down, click the SELECT button and pick your downloaded ISO, leave everything as is and click on START. Should Rufus ask you if it should write to the stick in ISO or in DD mode, pick the recommended ISO mode. Once it's finished, you have a bootable stick.

For advanced users, there's also the option of creating a so-called windows PE stick (a special windows version that can be run right from a USB stick) that allows you to run advanced scanning/removal tools like ComboFix and/or rescuing files from your hard drive from outside the infected windows system. For further details, see the Win10PE forums, and in-depth description on how to build Win10PE would be too much for this guide.

Anyway, back to scanning. You should now have at least one scanning/rescue stick ready. If your PC/Laptop runs on wired network, unplug the network cable for now. Plug in the USB stick, turn the device on and press the button that lets you select the boot device (in most cases it's F10, F11 or F12, see if your BIOS shows a note to launch boot select or the BBS menu). If you weren't fast enough and your normal windows system boots, press the reset button (if available) or do another hard shutdown via 5-second power button press.
When in boot select, pick your USB stick from the list with the arrow keys and select it with enter. Your live system should boot and guide you through the scanning. Depending on the size of your hard disk(s), a scan can easily take several hours, so sit back and let it run. Once the scan is finished, most tools show a report of what they found and if they could remove it.
If you wanna do another scan with a different tool, do so now. If not, unplug network again (if you re-plugged it during scanning for pattern updates) and boot into windows again. See if it still behaves weird and if there are any fishy-looking processes in task manager. If so, do another offline scan. If all looks OK, do a full system scan with your AV program and a scan with Hitman Pro for a second opinion. When both find nothing, your system is (probably) clean again. Though there is always a small chance that some part of the infection remained and might strike again later, so the only way to be 99.9% sure your system is clean again is to format and re-install windows.

If you got hit by a crypto virus and have encrypted files everywhere there is a small chance for a free decryption tool to be available. Go to the ID Ramsomware page and upload a rasom note or one of the encrypted files. The site can usually tell you more about what hit you and links to decryptors once they become available. If there's none available now and you don't have a backup of your files, you can keep a copy of the encrypted files somewhere. Sometimes, weeks, months or even years after a crypto virus spread researchers or police/security agencies might find a way to crack the encryption and provide a tool.