The old AMD issue. Try this -> http://msinfluentials.com/blogs/jesper/archive/2008/05/08/does-your-amd-based-computer-boot-after-installing-xp-sp3.aspx
Please don't write you wrote SP2. It can still be the issue.
The other cause I'm running into is malware and rootkits.
Bob
I have an Asus M4A785T-M/CSM Motherboard, AMD Athlon II, Windows XP home, I installed all my drivers then some windows updates along with SP2, After installation of SP2 I rebooted and it went into a loop of restarting, Windows would not load. This is rather annoying, I then uninstalled and re installed sp2, the problem remains, any ideas what could be causing this?

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