It's always good to keep a copy of all the .sys and config files, so that you can replace or repair broken ones manually whenever a boot process becomes corrupted. Though obviously you are past that point. Have you considered finding a copy of that file online somewhere, or from a XP installation disk?
Might be a long shot though...
Definatley try /chkdsk, and see if it will automatically clean up any errors.
Try Google for something like:
http://www.computing.net/windowsxp/wwwboard/forum/97354.html
http://www.hardwareanalysis.com/content/topic/31874/?o=0
It's annoying to sift through all the responses, but there's multiple things to attempt.
Have you made any large hardware or software changes recently, say a Windows automatic update, BIOS tinkering or new graphics card?
As a quick suggestion, make sure your HDD is #1 for boot priority, then hit f8 when booting up and start a command prompt.
Should be in C:\> by default. Try accessing the Windows directory of :
cd Windows\System32\driver\ if you can reach it, then rename MUP.sys to say MUP.bak, then reboot and see what happens.
If it lets you into XP, take a good copy of the file from the machine you're on, or from online and place it in the drivers folder and see if it happens again.
Could also try setting BIOS to default settings. And/or as a last resort, you can keep your data in tact and do as Reven said, by setting your boot priority in BIOS with CD ROM first, then doing a reinstall, "keep system files in tact" rather than reformatting. You will lose all of your custom GUI settings in XP, and probably have some conflicting or lacking registry information regarding the data already on the drive, (your data) but it will at least let you into XP(theoretically). From there you can decide what to do with your important files, then consider a fresh XP install.
