Do this:
Reboot your computer in a Clean Boot state.
It's possible a program or driver is causing this issue. To help troubleshoot error messages and other issues, you can start Windows Vista by using a minimal set of drivers and startup programs. This kind of startup is known as a "clean boot." A clean boot helps eliminate software conflicts.
How to run Windows in Selective Startup (Clean Boot)…
1. Click Start, click Control Panel, Administrative Tools, and then double-click System Configuration.
2. Click Continue, or provide Administrator credentials if prompted.
3. Click the General tab, and click Selective startup.
4. Under Selective startup uncheck Load startup items.
5. Click the Services tab, check the Hide all Microsoft services box, and then click Disable all.
6. Click Apply, OK, and then Restart.
Also:
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7