My computer des not shut down

hello
now this is a strange p[roblem…i use windows xp…and just recently my comp has stopped shutting down(neither does it hiberate, log off, stand by nor restart!).
I mean i go to the start menu n select the optio to turn off the comp but after clickin the the turn off button…NOTHING happens..its as if i hvnt clicked ny thing..besides this the comp is working fie…wats the p[roblem..?..some virus or wat??
waitin for a rply
Atish