Firewall activation

Question

My daughter and share a internet broadband connection with mine being the no 1 connection. For some unknown reason she recently turned off her windows xp firewall. I have rid her computer of over 20 infections (hopefully) but now cannot turn her firewall back on. The security centre keeps giving us an "unable to activate firewall" message. She is on-line constantly and we need the protection . Any advice would be greatly appreciated.