I'll preface this by saying that for the nastiest viruses, the best course of action time and frustration wise is to just reformat the computer. However, most common viruses and malware that infect machines today are pretty easily taken care of.
Preperation
Depending on the infection, you may or may not have internet access. Even if you do, it's wise to pull the computer off your home network to reduce risk of it infecting other machines.
If you have another machine, grab a small USB stick, and download the following applications to it:
Combofix (Direct Download Link)
Malware Bytes
SuperAntiSpyware
If possible, disable System Restore on all drives on the computer. Disconnect any external drives as well.
Cleaning
On your infected machine, turn it on and boot into safe mode. To do this, on most computers tap F8 during the initial boot screen and it should prompt you. If this doesn't work, check google for how to do it for your model.
Once into safe mode, insert your USB stick and drag all 3 installation files to the desktop. if you haven't already, disable system restore.
First, run Combofix. (Note: Some viruses will block you from running Combofix. Simply rename the executable to something like "Combo1" to get around this) You'll see a few warning messages, just click past them. Let it back up the registry. Don't install the Windows Recovery Console. You should then see the following screen while it scans:
http://img.bleepingcomputer.com/comb...owing-stag.jpg
If you see your desktop or taskbar blinking, this is normal. You might get the "Windows has started in safe mode" introductory message again, just click Yes so Combofix can continue scanning.
Note: If you have a rootkit on your machine, Combofix will pause and list the files associated with the rootkit. It will then reboot to clear the infection. Typically there are still many leftover files after this, make sure you reboot into safe mode and start over from the beginning and run ComboFix again.
Once Combofix has successfully finished scanning, it will generate a log for you. Reboot if it prompts you to, and go back into safe mode.
Second, install and run MalwareBytes. This is straight forward, and will clean any left over files that ComboFix may have missed. Once that's complete and it's removed all files, reboot if it prompts you to.
Lastly, run SuperAntiSpyware. You can skip this step if you think the first two steps cleaned the infection, but a little redundancy never hurts.
Cleanup
Reboot and log into Windows normally. Check to make sure that the symptoms you experienced before are gone. If they're not, make sure you had System Restore turned off, no CDs in your drive, no external drives, etc where the virus could have reinstalled itself.
Download CCleaner and run it. This will clear all your temporary internet files and also clean out your registry so that any left-over entries are taken care of.
Prevention
Always have an up-to-date virus scanner and all your Windows Updates. Obviously paid antivirus solutions like Kapersky and Nod32 are the best, but a good free scanner like Avast! is better than nothing. Don't neglect Flash/Java/Quicktime updates either, as many things use exploits in those programs to infect your machine.
Hope this helps.
Sono edit: STOP USING AVG ITS SHIT