If you are suffering from the continuous reboot problem that seems to be plaguing AMD users that installed SP3 then I can help, I hope.
I have an AMD 64-Bit processor, I was using 64-bit Ubuntu but I decided to switch back to XP for a little while. After I got finished installing things looked good until I installed my nForce drivers. After the reboot I couldn’t get back into Windows.
After doing some research it turns out this was also a problem when SP2 came out and somehow made its way into SP3 as well. The way I fixed it though is by renaming a file, c:\windows\system32\drivers\intelppm.sys to intelppm.bak. Unfortunately if you’re experiencing this problem you won’t be able to get into safe mode either. I had to boot into a pre-installation environment to rename the file. Once I did though I was able to boot up the computer without any errors.