when windows starts close all running programms,put your win installation cd in your cd-rom drive and do this:
start>run>type sfc /scannow and press enter .this will tell you if any of your system files are missing or corrupted.
this is for your information copy-pasted from windows help and support
System File Checker (sfc)Scans and verifies the versions of all protected system files after you restart your computer.
Syntax
sfc [/scannow] [/scanonce] [/scanboot] [/revert] [/purgecache] [/cachesize=x]
Parameters
/scannow
Scans all protected system files immediately.
/scanonce
Scans all protected system files once.
/scanboot
Scans all protected system files every time the computer is restarted.
/revert
Returns the scan to its default operation.
/purgecache
Purges the Windows File Protection file cache and scans all protected system files immediately.
/cachesize=x
Sets the size, in MB, of the Windows File Protection file cache.
/?
Displays help at the command prompt.
Remarks
You must be logged on as a member of the Administrators group to run sfc.
If sfc discovers that a protected file has been overwritten, it retrieves the correct version of the file from the %systemroot%\system32\dllcache folder, and then replaces the incorrect file.
If the %systemroot%\system32\dllcache folder becomes corrupt or unusable, use sfc /scannow, sfc /scanonce, or sfc /scanboot to repair the contents of the Dllcache directory.