Not really sure how anyone can give any real advice here.
The description of the issue is beyond vague.
Lag, are we talking about the incorrectly used term for seeing something on your screen that doesn't match what another player is seeing in an online game, i.e you shoot at a target that isn't actually there?
If that's the case then no chances are a virus isn't causing that.
Are we talking about stutter, i.e input lag, you push a button and their is a noticeable lag between your input and the action occuring, in which case yes viruses can cause that (but if it's a high end gaming rig the chances of a virus being so intensive that you would notice it are slim)
Are we talking about slow down or some other performance issues? Yes again a virus could cause this but again we are talking about a virus that would have to be so system intensive that you would notice it in day to day use.
To be honest we need some more info
1). A better description of the issue, does it occur with all games, certain games, one game, does it happen all the time, at the same point, during high demand gaming sections, does it occur only in online games or offline games or both.
2). We need to know what you're running, what's the hardware you're using and what OS is it running on.
3). Have you made any changes to the system, you said the system used to work but has issues now, have drivers been changed or updated have you changed any hardware.
