Our XP SP2 Media Center machine (bought used a while back, mostly used for games and email), got itself into a rebooting loop. I have seen machines with power supply trouble do this, but this one has a nice Enermax. So my first Q was whether it was a hardware or software problem. I booted on a Xubuntu 8.04 hardy-desktop-i386 CD with the "don't affect your machine" option, and it ran fine for half an hour, so the finger pointed at software.
The XP boot menu offered "last good config" and we tried it, but that didn't help, still cycling into reboot. The XP boot menu offered Safe Mode, but it hung at MUP.SYS.
We made an "XP boot" floppy per instructions I found online (look for 10_things_winxp_boot.pdf). I formatted the floppy on our XP Pro machine, and copied boot.ini, NTLDR, and Ntdetect.com from C:\. That would be an "XP boot" floppy for that machine. However, Boot.ini is machine-specific, so we needed to copy the one from the ailing machine. I renamed the Boot.ini on the floppy in case I need it later for that machine.
Xubuntu was able to mount the NTFS harddrive, which it showed in the device list as a SCSI drive even though it is actually IDE, and the floppy. SUDO was needed to do the mounts, copy, and unmounts.
I checked the boot.ini by eyeball on another machine with Quick View (it's plain text), it looked plausible. Removed the Xubuntu CD and booted via the floppy. Chose Safe Mode from the XP boot menu, but it actually just booted normally. All Programs/Accessories/System Tools/System Restore, chose a restore point about 3 days ago, and that solved it.
I figured I should post this for other people who haven't known about making an XP boot floppy ahead of time.