Help me!!

hello can u guys help me my resolution is on 1024x768 and on 32-bit windows xp i have a problem when i go on game its fine and i join room its fine then when i click join the match like the real game where u kill when it fills up the maps bar 1/4 the screen turns black and goes into a small square on the corner of my screen for 30 seconds + then it comes back to normal and then loads up can someone help me please please im begging u help me with this solution its ****in me off like hell