System automatically reboot when startup

Hello,
my name is ashok kumar, in my system have installed windows xp, and also when that time update my antivirus software then that time my system will reboot automatically when startup. in this problem occured in spyware anti virus and f-securfe anti virus also. iam not try to another anti virus s/w's.

so could you please to reply me the best solution for this problem.

thanking you,
Ashok.