Avast is one of the best and is free. We have used it for years and even with new laptops that came with Symantic, we also installed Avast. Avast caught viruses that Symantic didn't.
You are having problems because that version of Windows is ANCIENT. You will have an extremely hard time finding anything that supports it. Antivirus software must be re-written each time a new version of Windows comes out, so the old versions are tossed out. Also, each time there is a new virus, the software must be updated, and most companies won't bother to update a really really old version of software than almost no one uses. So while you might be able to find some virus software that you can download, it probably isn't up to date on new viruses, so it's often a waste of time and probably money.
You would be much better off just updating the computer to Windows 7 ... which probably means you need a new computer. A computer that old probably isn't compatible with new versions of Windows either. Yup, it kind of sucks, but that's they way the computer world works. And as far as viruses, if you are just using it to send email or go on websites that you know, you don't really need to worry about it. Don't download games, don't go to any questionable websites, and don't download music or movies and you probably won't get viruses.
