agreed with all. add another 512MB RAM (to get dual channel also)
and format, to get rid of the crap Windows has accumulated over the years.
also, if you are okay with it, try overclocking the CPU
Use the System Configuration Utility to speed up your system and your start-ups. Click Start, Run, 'msconfig', Startup tab. Uncheck everything but what you absolutely must have (firewall, AV, graphics drivers, whatever). Click Apply, OK, and reboot. A grey box will pop up on startup. Check the Do Not Run on Startup and you're in business.
Try those and see if it doesn't help out your rig.