Laptop freezes

My laptop freezes on windows XP after I’ve entered my username and password. We’ve tried fixing this by formatting the harddrive and reinstalling the OS but in vain. We first installed windows 2000 then XP. We are, however, able to logon onto the safe mode. Could this be a hardware problem? If yes, what exactly? If there’s a hardware diagnosing software that I can use, how will I install it since I’m only able to logon safe mode and not normally?
Please come to my rescue.