It might be related to the startup programs.
Disable every software and service come's up with Windows (Leave only the necessary stuff)
Related information (Guide) you'll find here:http://support.microsoft.com/kb/310353
Also be sure that your PC is clean from Malware and viruses.
Scan your PC in "safe-mode with networking" (use Malwarebytes' to scan for malwares, and good Anti-Virus [like Avira for example] )