OK. First of all , make sure you haven't overclocked anything. (i.e "EVERYTHING" should be reset to standard stock default settings).
Now we start step by step.
Judging from "the way" you presented your question , I'm taking a blind stab in the dark that you are not very familiar with how computer systems , software , and Operating Systems work.
Of course , if I'm wrong , make sure you TELL me so I won't waste much time with preliminaries.
1) Boot into Safe Mode. (Press F8 during POST (booting up)).
The majority of cases of windows failing to boot is A) Driver conflict B) Hardware issues such as recently replacing CPU or Graphics card.
2) A) If you sucessfully booted into windows in Safe Mode , then its almost certain that its driver conflict.
B) If booting into windows using Safe Mode is also impossible , then most likely, the problem is hardware related. In this case , it is advisable to actually do a FRESH install of windows AFTER reformatting. That's right. You can't simply do a "repair" of the windows installation by running the setup utility. It has to be a CLEAN install.
3) A) After booting into windows in Safe Mode , use the process of elimination. Acitvate a driver or software you installed ONE BY ONE. Start with the most recently installed. OR you could "uninstall" your most recent program/game. OR you could use the "System Restore" utility in windows XP to restore to a previous working state.
OR ... if you are comfortable with the inner workings of windows , then you could also tinker with the "Services" with the msconfig utility. If you don't know what msconfig is or where to find this little utility , then don't even bother trying.
OK ... that's all for now. It's got kinda long.
Keep us updated. Others may fill in the details with your next batch of questions.