What you can do is download a program called END IT ALL. Do a google search (enditall). This will kill any unnecessary processes and programs. If it is caused by a memory leak from another program, it will solve your problem (temporarily). Does the computer have onboard video and audio? If yes and you have external audio/video card installed, make sure the onboard are disabled.
If you do locate the program/process causing the lag, you can go in the config system (hit run and type msconfig.exe) select setup and disable the program from loading at startup. If it is an adware, use adaware (google search) from lavasoft and scan your computer to remove it. You can also run spybot search and destroy as these two programs complement one another real nice like.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO