Does it do that in Safe Mode? Please try this next one even if you don't have your OS disk. If it asks for the disk during the scan, we'll know there's something wrong with the files.
Do you have your OS CD/DVD?
If so,
1/ Click the Start button.
2/ From the Start Menu, Click All programs followed by Accessories.
3/ In the Accessories menu, Right Click on the Command Prompt option.
4/ From the drop down menu that appears, Click on the Run as administrator option.
5/ If you have the User Account Control (UAC) enabled you will be asked for authorisation prior to the command prompt opening. You may simply need to press the Continue button if you are the administrator or insert the administrator password etc.
6/ In the Command Prompt window, type: sfc /scannow and then press Enter.
7/ A message will appear stating that the system scan will begin.
8/ Be patient because the scan may take some time.
9/ If any files require replacing SFC will replace them. You may be asked to insert your Vista DVD for this process to continue.
10/ If everything is okay you should, after the scan, see the following message Windows resource protection did not find any integrity violations.
11/ After the scan has completed, Close the command prompt window.
