follow with care:
Place the Vista CD in your DVD drive, after a normal boot.
When prompted to "perfom upgrade" Choose this option.
Do not choose the re-install, as this will overwrite your programme file's
Takes a while but it will re-install a clean copy of Vista, .
Could also try the following:
Shut down the computer.
Re-start, and press th "F8" key
Choose the option "last known configuration"
Safest one first
Ie: the latter