Teen Geek's solution will, of course, work but you'll then be forced to use a 3rd party program for Zip files from now on instead of the functionality within Vista (which is easier to use). Also, WinRar is shareware whereas 7-Zip is freeware so you may
want to choose the latter. You may want to fix Vista's zip functionality instead (or in addition to):
Chances are good that some system files in Vista have become corrupt.
Then let's check some of your system files:
Go to Start / All Programs / Accessories / Command prompt and right click on command prompt and click run as Administrator.
Type sfc /scannow and enter and let it run. It will scan and try to fix some of your system files. Hopefully it will complete with no corruption it could not repair (if there is such corruption post back here or try to analyze it to find the problem file(s)
While in Command Prompt, type chkdsk /f /r and enter and let it run. It will scan and try to fix any corruption or bad sectors on your hard drive and mostly remove that as a cause.