I had the exact same problem. I just build a new system and installed a fresh copy of Windows XP Pro (not a vendor disk Microsoft disk). I hadn't got a chance to install any software on it yet. It was booting and running fine. Then I downloaded and installed SP2 and it rebooted and froze at the Windows status bar. If I try to boot into safe mode it locks while booting. So I insert the Windows disk to try to do a fresh install or repair and it restarts itself while initializing the setup. I don't know what I'm gonna do now other than formatting from a startup disk and then booting to the CD. I also had my share of cussing out Bill Gates last night. I'm gonna keep fighting it but any input would be appreciated.

Pentium 4 2.8E 800 MHz FSB Prescott
Soyo P4I875P DRAGON 2 v1.0 Black Label
Ultra 512MB DDR RAM
Mad Dog 128MB Video card.