About the only upgrades would be RAM (although 512 should be sufficient) and/or a larger hard drive (how much free space do you have?).
I would first off suggest the following:
As a first response you might want to post a HijackThis log for review by our Pros in that section:
Please follow the instructions
HERE & then post your log in a new thread in the
Spyware, Adware, Viruses and HijackThis Logs section.
(Not in this section please).
Please specify what issues you appear to be experiencing along with your log.
After you are all cleaned up install SP2 and all subsequent Critical Updates from Windows Update.