One suggestion: Let us know the brand and model of the laptop, the version of Windows (probably XP, but things end up in the wrong sub-forum sometimes) plus any messages that it shows immediately before the startup.
As soon as the BIOS sequence completes and before Windows starts (you may have to play with it to find this point), press F8 and select a Safe Mode start. Let us know what happens.
If I had to take a flaming guess at this point, your installation got corrupted and you may have to do a repair install, but let's not jump to conclusions.