Depends, if you're getting fps lag (your fps are going low, able to check top left of the screen), try using programs such as GameBooster, turn off all the programs you're not using at the moment, preferable delete "explorer.exe" by processes manager (should've help on XP, no ideas about Vista and 7)
If you're getting high ping, well, might be caused by downloading something in background, if you're sure you're not downloading anything (including anitviruses updates), problem might be your connection speed. In that case, try to fix/ubgrade your connection speed which ofc costs money. As for me, my average download speed is 100/150kb/s which makes lags, on server hosted in US or elsewhere away from Europe, reasonable.