U can have your videocard's driver updated, add more RAM and/or Overclock your Processor. But if you want real performance you'll have to spend some serious cash for a new PC.
1. uninstall the programs you very rarely or never use
2. disable all the programs you don't need while gaming (windows media center, java update, etc...)
3. use task manager to End Task the explorer.exe ( you can restart it via the task manager by selecting "run" and tpying in Explorer.exe or pressing the windows key twice)
4. while you are still in task manager, stop the services you are not using (Bonjour, Apple update, ...)