The free way is to defrag your hard drive, uninstall any programs you do not need, run virus check, or if you are brave, format the hard drive and start again (bearing in mind all your information will be lost if not backed-up)
The non free way i.e. sytem upgrades is to increase memory size, Graphics card, CPU+MoBo. It all depends on what your main use of the PC is!
Use "Advanced Uninstaller 2004" this baby is gold trust me it cleans up everything and even speeds up the registry. Also I recommend AdAware, Norton System Works and a once-a-month (or week) defrag.