Well first off can we have all the specs for the system? Second, are you running any programs in the background? Third, have you messed with the exe or do you have any third patry mods? Fourth, have you run a virus scan/spyware scan?
Do you have the latest version of DirectX and your graphics drivers? Do you have an internal sound card, if you have a crappy sound card it may actually be the number of channels the sound is playing on rather than the graphics.