Lower the game resolution.
If you run Windows XP, click Start, RUn then type: MSconfig.
Hit enter. Under start up tab, uncheck some un-neccessary programs when windows load. For example: Graphic related, Chat programs, and many more. Just keep only security softwares check. Remember you un-check it doesnt mean you delete them, just that you tell the Windows dont load these programs unless you say so. Hope this help. Oh defragment your HD is big help too.