Ok try booting up the computer in safe mode.
Whilst there click start - run, type msconfig and press enter.
click the diagonostic startup.
click apply and restart your computer
Doing this should only load up the basic services and devices required by windows. If this works upon restart open up the msconfig control panel again and select selective startup click process system.ini file - apply and restart. Upon restart open up msconfig and click process Win.ini file and restart.
Do the same for the other 2 files there.
This way you will find where the problem lies
If it turns out to be one of the startup items then click the startup tab and disable all.
Enable them one at a time restarting the computer after each one.