I am not a computer techie, but I had a similar problem a few months ago. Computer would turn on, go through the usual BIOS screen, then hang and reboot itself endlessly. Finally, it did tell me that Windows could not be started because of a missing or corrupt DLL (dynamic link library, IIRC) file.
So, after googling the DLL in particular, I finally came upon a site that dealt with repairing the problem manually. I had to find the WinXP install/recovery CD and boot into DOS. After that, there were a series of commands I had to type in order to tell the computer to locate, unpack, and install the DLL I needed from the CD. I think there were about 6 or 7 commands. Turns out that after I did it for that DLL, the next attempt to boot reported ANOTHER bad DLL file, and I had to go through the process several more times.
Ultimately, after unpacking and replacing several of them, Windows started and everything was back to normal. I immediately bought “WinTasks” and a firewall, as well as using Spyware/Malware stuff each week. Also got rid of Internet Explorer in favor of Mozilla. Since then I’ve not had the problem again.
I realize this isn’t a heck of a lot of help, but maybe it’ll count for something. Good luck!