XP won,t boot?

Each time I try to boot my window XP,it runs a system and folder files repeatedly,with the result that the files are okay.But,the window won,t appear,instead,it re-boots again.I have two other op sys,98se,and 2000 pro,installed.When I go to the other op sys,it boots normally.What could be the problem?Am I infected with a virus or should I re-install the XP?