What do you mean by "lagging" exactly?
Is there STUTTERING, or is the frame rate simply lower than what you expect?
You say "video card is installed but still lagging"; does this mean you have been making CHANGES to your system and it worked fine before?
Usually hardware is either working fine or it starts to cause crashing or corrupted data. What you describe sounds more like a software issue.
Anyway, before you proceed much further:
run MEMTEST for at least an hour ( www.memtest.org )