help with lag

i ahve a pc that i built and hav had for 2 years and now it lags on games. even single player games. i have done all that i know to do. i have ccleaner, advanced windows 3, panda, and i have a gigabyte brand motherboard, nvidia geforce 8500 gt 1gb, 2 gigs of ram, 160 gb hd, and a p4 processor, new 500 watt power supply, and this is speedfan.