--First thing--:
Maybe you have a virus, thats probably the most direct answer to your question. Run a free test at here , download the trial norton or run a scan from their site.

--Second Thing--:
Has your comp done this before? If it only recently started doing this follow these steps.

----IF YOU HAVE WINDOWS XP-----
Start-->All Programs-->Accessorys-->System Tools-->System Restore.

This allows up to back track your comp to when ever time you had the last Restore Point Done.

Such as if Today I had a problem i could go back 2 days where i didnt have the problem, and my originaly files that exisited on that day restored.

Your computer might make check points automaticaly if the computer is on at a given time. If not you may be forced to back track 1-30 days to find a restore point.

If this confused you or it is confusing pm me and ill try to help u out more.

---Third Thing---
Have you made any software or hardware changes? That may be causing your problem. (driver, ect)

---Final Thing---
You Might be screwed if you can't find out what happend to your computer. If you have Windows XP then you might still be in luck. But anywho you might have to do a reboot of your computer.

Advice: if you have more than one drive, move all your anime to the secondary drive and not the main (C:/ <---generaly the main drive)
and what ever files you might need, and hope that only a C: drive format is required.