Serious lag issues with Deus Ex: Human Revolution
First - update your video card drivers. And Direct X. This can help. Also, upgrade all the drivers on critical systems. Then, if this doesn't help, lower all the settings - and try to raise them one by one; maybe you'll find something that makes your PC lag.
You could try to Ctrl+Alt+Del after launching the game and monitor how much CPU and RAM the dehr.exe process "eats". Maybe it's not game's problem, maybe you just got something running in the background and hanging your system, or possibly some kind of virus.