I'm assuming you're using XP which is already pre-installed? I'd definitely wipe that off and clean install XP if that's the OS you want to use.
After installing XP, the very first thing you should do is install SP3 (Service Pack 3). You will almost certainly need to download and install the latest Windows Update Agent before Windows Update will run. Install all the updates you need, then apply the EPOS registry tweak and run Windows Update again for the latest of the XP updates.
I really wouldn't bother bogging down the system with a firewall, antivirus or malware scanners. If you're sensible you'll be fine.