How fast is your computer?
I would suggest format and reinstall windows originally
But you can do some windows maintanence, get a virus scanner (AVG is a good one), adware/spyware remover (ad aware and spyboy s&d) and finally get windows antispyware, which has a built in firewall as a bounus if you don't already have one
You can download all of those from download.com and microsoft homepage, but make sure you update and download all definitions before you start scanning
Next you should get ccleaner, which i use because its probably the best .tmp file cleaner, as well as a registry cleaner, you can download this at ccleaner.com
Also, since your computer is old, type in 'msconfig' in run and then take off all startup programs except firewalls and other programs such as objectdock, etc
Next, defragment it, which helps alot
Hope that helps lol