For the past 3 days my HDD has been constantly clicking away in the background, even with no programs running, right from startup. Its causing slowdowns when I run programs, especially when first starting them up, but also occasional spikes during use. According to Task Manager, the CPU is barely being used and only ~30% of memory is being used, but disk activity is very high. Any program running will rack up huge i/o reads and/or writes - after an hour or so, firefox.exe has racked up over a gig of i/o write bytes.
I first tried a system restore to a few days before I noticed any problem, with no effect. I've used windows to run memory and disk checks; no problems come up (disk check had to be run on startup due to 'drive in use' always coming up within windows). The hard drive is only ~1/4 full, and has regular defrags scheduled. I've disabled parts of McAfee with no effect, so its likely not my antivirus (combined with pretty safe internet use, I doubt its a virus either). I've done some googling, but haven't found an easy solution. My next steps are 1) complete re-install, then 2) buy a new hard drive and migrate everything over. Before I do those, I was hoping someone might have an educated guess which would be more likely to succeed, or anything else to try.
Computer: almost 4 years old, Dell PC w/ Intel i7, 8GB ram. Whatever relevant info I can provide, let me know.