Windows xp internet problems

I have no clue whats wront with my pc but everytime i go on the net with it.....its really slow....and lets say im on msn messenger it just logs me off of it.....or i have trouble logging in ....and say i want to view any webpages.....it takes like an hour for it to load....or it just doesnt load at all...i have highspeed internet so i doubt my isp is the problem.....could it be a virus of some sort.....im really stumped on this 1 ...i also tried rebooting my comp......many of times....and 1st it seems like the problem is gone but like 15 mins later its back again ....can sum1 plz help me out.....this is driving me crazy !!!.