The following are all good
free tools for keeping the computer clean and free of malicious code as well as other nasty stuff.
CCleaner
Malwarebytes Anti-Malware
Revo Uninstaller
Avast! Antivirus
Zone Alarm Firewall
My computer is just as old, if not older, than Paven's. It has an 3 Ghz Intel Celeron processor and runs Windows XP. I use Firefox 3.6.18. So far, it is stilling running fast and pretty much issue free. The key here is
proper maintenance. Some of the tools I mentioned above should be run regularly. Once a week. Once every two weeks. Whatever you choose, stick to it. You will avoid a lot of hassles. Do not let the stuff in the temporary file areas and caches accumulate.