Hi. I'm brand new here too - and I have exactly the same infuriating problem. I imagine I must have some kind of trojan. The main problem I had encountered Thursday, when I updated last updated the Avast exe. It asked me to reset the machine after installation, which I refused to do because I had other tasks online I wanted finished with. I waited approximately half an hour and then reset it, whereupon I was greeted with the XP boot failure prompt and a subsequent automated reset. I tried several times to gain access, whereupon first the ntfs.sys driver failed into a reset, then the Mup.sys driver, but eventually managed to get booted up using Safe Mode with Networking - how, I don't exactly understand, as it seemed completely random to me at the time.
Anyways - to cut a long story short, I found I had several viruses on running a full Avast scan in safe mode, but unfortunately do not recall their names (grr), moved them to the chest (as suggested) and (assuming that all would be fine, henceforth) continued to reset XP into normal operation. However, after shutting down this session, I returned after the weekend's inactivity to the exact same problem encountered previous - except this time, there's no entrance permitted into the OS, as the machine resets upon encountering Mup.sys on the safe mode's command-line interface, or just a few notches along the progress bar (in normal bootup) and then a reset.
To compound this problem, I can't repair XP right now because I've mislaid the setup cd along with my caviar drivers . . .
Any more information regarding this issue would be greatly appreciated, thanks! Although I do expect it could be fixed by an XP setup repair on certain system files, together with another scan with a new Avast update . . ?