You haven't given us a lot to go on, e.g. specific program or activity playing up!

Have you installed all the available updates?

If not, that's a good place to start.

If you have installed them, try the system file checker:
Click Start > All Programs > Accessories and right-click Command Prompt, then ‘Run as Administrator’. Type sfc /scannow (note the space) and press Enter. This may take some time and you may have to put the Vista DVD in the drive.
You can analyse the SFC log file, here http://support.microsoft.com/kb/928228/en-us