Avast is pretty gentle but I think they went to 2000+ support only about a year ago.
You can set the AVG/SB to scan that partition but it wouldn't offer you any sort of real-time protection while you're using the 98, and again, you can't be sure you'll get any 98-specific bugs or any bugs to which 98 is vulnerable but the software doesn't have the ability to detect. I mean, who knows, there might be antiquated definitions in there, and you might have a certain degree of immunity as well if the bug is looking for a key or file 98 doesn't have. I guess if it's free you can always try installing it and seeing what happens.
You won't get any current version of Ubunutu running on it.
Even the stripped down installs (Lubuntu/Xubuntu) require 5GB, 512MB and a 500Mhz processor, and those are the min specs.
If you can get the RAM up to 512 your options open up a bit. Under that you've got limited options: Puppy, SwiftLinux, TinyCore, CrunchBang and probably a few others. Processor speed for you is also a consideration. You can always just start ripping your distro of choice up and try to MAKE it fit and run (and behold, a distro is born!
There ARE anti-virus software for Linux, and Linux IS vunerable to bugs. BUT the general answer is your risk is much lower since most bugs are targeted to the much larger potential victim pools provided elsewhere.