2000 SP4 is fine if you just use 3rd party firewall/av. Its a rock solid OS, and even though MS won't support it anymore it doesn't mean its no good, and how often do we actually call MS with a problem?
You will stop getting updates/patches after they quit supporting, but if you use a browser other than IE, mail other than Outlook, use an AV and a Firewall you should be pretty safe.