As already noted, reinstalling the program is the best way to get it back in order. If possible, uninstall, defrag, etc before reinstalling. If the program doesn't appear in Control Panel|Add/Remove Programs, Reinstalling and then Uninstalling from Control Panel is usually the surest way of getting all the Registry entries cleaned up before you do a final reinstall. If you reinstall and it still doesn't show in Control Panel|Add/Remove, there should be an "uninstall" in a Photoshop (or maybe Adobe) folder in C:\Program Files, but Adobe products usually only put a .dll there to tell Control Panel how to do it. As a last resort - after careful backup of data - you may have to just delete the Photoshop folder.

Note that your installation disk may have uninstall instructions, possibly in a "readme" file; and you shouldn't have to have the program installed to browse the disk(s).

A second on the recommendation that you backup - especially your graphics files - before doing major surgery on your system. And don't forget the .psd files (and .pdd if you have any), which I believe are big Photoshop's "native" format. Anything you may have saved without flattening (ie. anything with layers open) may have defaulted to a .psd, perhaps when you intended to save as something else.

Photoshop is a real "memory hog," for both RAM and drive space - especially for temp space on the HD. It's possible that you've just gotten the drive it works off of too full for it to find space for it's temp files. Defrag and check whether contiguous free space looks reasonable, if you think this might be a problem.

John