I have the same problem 3 months after buying my sony vaio with XP and ATI 9700. Gets past XP load up screen and goes blank - have to use reset button. Works in Safe mode and finally discovered that XP would boot up ok if I disabled the ATI graphics adaptor. After boot up, I can enable the ATI in device manager and everything works perfectly. I disable it before shutting down so that windows won't crash next time around. Still don't know what suddenly caused this to happen. Had recently installed Half Life 2 which crashed early into the game ? System restore didn't help. Odd indeed!

Have you checked Windows Update to see if there are any newer XP-certified drivers since XP was released?

Have you downloaded XP Service Pack 2 at around the same time that the problem started? Note that you might not have actually realised that SP2 has been downloaded as it's been happening in the background on some PCs without the users really noticing - you can check by right-clicking on My Computer and going to Properties.