It sounds to me like you have a virus or some other kind of maleware, that has either damaged your Explorer.exe or has taken control of it. First, I would do a full virus scan (if possible...if you cannot due to this issue, then go to the next step).
Next I would fix the Boot loader, Here are instructions to do that:
http://www.fixmyxp.com/content/view/160/129/Then if that doesn't work, do a repair install....here are instructions to do that:
http://www.fixmyxp.com/content/view/32/129/