First, physically disconnect from the Internet to prevent a malware infection while your Anti-virus and/or Firewall are disabled in the following steps, then do a normal boot:
Click on Start then Run, type msconfig and press Enter.
Click on the Startup tab and disable everything (I mean everything).
Do a regular boot, see if it runs normal.
If yes then use msconfig to enable several items at a time till you find the culprit.
If no, click on the Services tab. Check the Hide All Microsoft Services box then click the Disable All button.
Again, do a regular boot, see if it runs normal.
If yes then use msconfig to enable services till you find the culprit.
Once you've found the culprit, uninstall it or find out how to eliminate it from your system. Simply disabling it in msconfig is a temporary fix at best.
Reconnect to the Internet when your AV and firewall are active again.