Defrag the system, use Ccleaner and remove temp files, isolated registry files, scan the computer from a virus guard, update the windows ,update the virus guard, remove unnecessary software such as larger heavy software that you don't use. don't use multiple virus guards, install the latest drivers (such as network ,graphic,chipset,sound)
uninstall software which may impact on performance of your computer,monitor the system performance through administration tools on control panel such as hard disk read/write rate, processor utilization, etc..