My friend's computer is lagging him in games... is there any website or application he can download that he can use to test his computer to see what his bottlenecks are and what equipment is causing him to lag?

uhm justtt defrag the harddrive? that always helps :)

He alreaedy tried that.

You can download diagnostic tools for this sort of thing, but all the free stuff I've tried in the past is usually full of trojans.

I usually just defrag the harddrive and make sure absolutely everything is updated. He should check his graphics card website for recent updates etc.

http://cyri.systemrequirementslab.com/srtest/

Microsoft has a computer perfomance monitor wizard ,download that.

What are the specs on the computer, and what games is he playing? It could always just be that the computer isn't up to the task, really.

My first reaction would be to start the task manager to see if there are any processes eating up a lot of CPU.

First question, is it a laptop or PC? if it's a laptop make sure that the GPU core temperature doesn't rise above 50, that will cause massive lag spikes on any game, even solitaire.
The same applies to PC, but the temperature spikes are less common.

If not, the diagnostics tools given here are good enough.

If the problem started recently and suddenly, something was installed and is running itself in the background, eating up CPU cycles and memory. Check the task manager first. The "lag" should only be noticed for tasks that previously ran smoothly if that's the case.

If so, roll back the registry with system recovery during start up to uninstall whatever is currently hogging resources.

Worst case scenario if some unwanted malware code/app is running in the background and system recovery can't remove it, back up, reformat HDD, reinstall OS, reinstall apps. Hope that's not the case.

Commercial system tools like Defender Pro or System Tools often identify common problems and can optimize the operation of your system.

If HDD is defragged, check that box.

If you are running current VGA drivers, check that box.

If it's a new game(s) he's trying to run and his computer is "lagging" as it's already been said, the specs of his system may simply not be up to the task of running at the settings he wants. Time to upgrade hardware or run at reduced settings.