A few initial thoughts.
1. It could be the video card. Tap the F8 key just prior to the XP Splash screen but after the BIOS screen. This will take you to the boot up options for XP. Select "VGA MODE".
If it comes up in super low resolution then you have a driver problem of some sort which can be fixed from this mode.
2. Can you boot to "safe mode"? This is done by tapping the F8 key (or sometimes F5) during boot up then selecting "safe mode" from the options.
Also look here for these and more detailed experiments to hlep locate what's gone wrong ....
http://ask-leo.com/why_does_my_scree...en_i_boot.html
Post back again to let us know how you get on. If nothing here works other suggestions will no doubt be made.
OJ