Need more information... Let's get a bootlog generated... See this page (http://forums.cnet.com/5208-6142_102-0.html?threadID=26794) (or any number of other pages google will cough up if you search on "xp bootlog") to start bootlogging. Once you get a bootlog, have a look at it and maybe you can see what program is causing the problem as it tries to load. If you want to send me the bootlog file, feel free. Or post it here if you feel comfortable with that.
Another thing you can try, if you have your XP CD, is open a command window (Start > Run > type "cmd" in the box) and then type this at the command line: sfc /SCANNOW
That will compare all the system files on your machine with the system files on your CD and (supposedly) fix them. This could be a long process and the results might depend on where you are in the Service Packs, etc., and the problem might not even be with the system files so I wouldn't try this until you've had a look at a bootlog file.
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Another thing you can try, if you have your XP CD, is open a command window (Start > Run > type "cmd" in the box) and then type this at the command line: sfc /SCANNOW
That will compare all the system files on your machine with the system files on your CD and (supposedly) fix them. This could be a long process and the results might depend on where you are in the Service Packs, etc., and the problem might not even be with the system files so I wouldn't try this until you've had a look at a bootlog file.