Hey I just had that same problem. Unfortunately for me, I had to reformat my C drive. After several hours on the phone with Dell Tech Support and several different attempts at fixing it, that seemed to be my only solution. I would get the dell logo, it would go to XP, a would get a quick flash of a blue screen (which I was able to read that there was some kind of fatal system error) and then back to dell logo again. I didn't want to reformat and lose everything, but..
Have you tried starting in safe mode? Good Luck - hope you can find an alternate solution