Especially for your library computers you should "bone up" on what the legal situation is for your location. Some legislatures have been attempting to "help," and the situation is somewhat different depending on where you are. (Google spam law your country/state perhaps.)
Just remember that the standard definition of FUBAR is "a SNAFU that receives management (legislative) assistance."
WinXP, if kept updated and with a couple of the "add ons" invoked, does include some spam and malware blocking features that are not too intrusive, and seem to be of some help. The "Windows Defender" thing does appear to help, but it requires verification of your licenses - and it's still for now at, or just coming from, "beta" support status. You probably won't want to rely entirely on it alone, but it may (or may not) help, depending on what else you use. It's probably more use to catch/remove the most critical malware crud than just as a SPAM blocker.
Although email as a source of real crimeware seems to be less in favor than in the past, most of the same kinds of crap that your library patrons can (innocently?) put on your system still can come from SPAM. A system like a library can spread something to lots of people unless general malware defenses are better than average. It probably would be well to consider a good "server quality" commercial protection suite there, for SPAM and all the other kinds of "annoyances." (It won't be free.)
How bleak the situation is? Spam Never Dies (Just to lighten your day).
(The page linked had a minor "page defect" a few hours ago. I've sent a "piss on you" note to the webmeisters. It shouldn't interfere too much with reading enough to get the point.)
Your own WinME can probably limp along if necessary and if you're careful, but WinME (and of coure Win98) are "out of support" and will not be able to close new holes as the crimeware people find and exploit them. Be warned that vulnerability will not ever be reduced there. Even Win2K is rapidly approaching "unsupported" status. Eventually you'll probably be forced to get a newer system.
If you are "personally involved" with your library machines, and especially if the library might be interested in a "high hor$epower" fix, you might(?) find some help at ZD White Papers Library. The "white papers" border on blatent advertising in many cases, but wading through that kind of crud is largely how one finds out what's going on, what's available, and what it can (and can't) do. (They may ask you to register. I probably have a cookie from them, so I can't tell if they look before they open.)
Sorry I can't make any personal recommendation of a program that will solve all your problems.