Did the system reboot successfully after the SP2 installation? I've seen this happen with bad SP2 installs. However it's usually only happend if the system is infected with spyware when you do the SP2 install (not likely on a new install) or if you have a bad driver. Some older XP drivers are not SP2 friendly.
Try disconnecting all your USB devices (I've seen a misbehaved printer driver do this after an SP2 install). You can also try pulling all cards except the video card and see if you get lucky. If you do get it to boot, once you identify the problem device, uninstall it's drivers and utilities and download the latest drivers from the manufacturer's web site.
Another shot in the dark: Try "Last known good configuration...." from the boot menu. Should restore the registry to the condition it was in the last tiem you succesfully booted.
Failing that you'd have to do a manual registry restore which is a pain. Since it's a new install a re-install might be easier. Here's the instructions if you want to try it:
http://support.microsoft.com/kb/307545
BTW, the above procedure is MUCH less time consuming if you use a BartPE boot CD or connect the drive as a spare in another system, but I don't know what your tech skills are.
If you do perform another clean OS install, do yourself a BIG favor and DO NOT install any USB devices or printer, and pull all cards except the video card until AFTER you get SP2 installed and THEN install each device ONE AT A TIME, and make sure you download the latest drivers. It'll go smoother with less chance of this happening.
Hope this helps...