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First course of action, try starting your computer up in 'safe' mode.
Assuming you run Windows 2000 or XP or later, when your computer is booting up, press F8 repeatedly until the boot process is interrupted and Windows asks you how you would like to start. Highlight (move arrow keys up or down) '1.Safe mode' and hit 'Enter.'
Try to access your files in this (safe) mode. If you can, it might be a virus or something. There is hope. If you can't, it's Computer Aids, like matt says. There is no cure. Contrary to popular belief, CAIDS is not a result of too much porn, but may result from sharing of needles with infected computers.