It saying speedhack

Hello, i have never used any hack in crossfire but yesterday it started to happen a really weird thing. When i have played my match and the match results comes up it always come up a little window that says: "Speedhack detected"
OK!
when i click OK or the X the game is quiting and whn i try to restart the game it come u a blank small window and internet come up with gameguard.. plz help me, i must restart my computer just after 1 match.. HELP!!