Windows XP will not boot

I recently answered a call for a customer that was having a problem with XP home edition not booting. The OS load would attempt to start on both a cold boot and soft boot. The machine would repeatedly fail to start and in fact would continue to restart. I discussed this problem with fellow workers and we were stumped. Since then, I have seen this problem on 5 other machines. I have a feeling that I am overlooking something extremely trivial. No form of recovery was able to work, i.e. system restore. The machine would not load LKGC or safe mode drivers either. No joy from tech support for each particular machine either. Any help would be greatly appreciated.