There may be a "removal tool" on the demo disk itself - or at least a "readme" file with instructions for how to do it.
In Win98SE a "normal" anti-virus installation will show in your Control Panel Software listing.
Start - Settings - Control Panel Select the "Add/Remove Software" If the McAfee did a "normal" installation, it should be in the list that displays. Click on it and then on OK, and it should be removed.
If the program doesn't appear in the Add/Remove list, then the demo disk probably had its own "install" program, and didn't use the Windows install process. Open Windows Explorer and select Tools - Find Files, or just use Start - Find - Files and put something like "McAfee" in the "what to find" box. The intent is to find the folder where McAfee installed. Look in that folder for an something named "uninstall" or some cute variation thereof. (unadvised.exe is sometimes used, but if you right-click the file and look at properties, you can often see something about what it's for.)
Your 20 minute "hang" indicates that the trial program is probably doing a full system scan at each bootup, and is doing it before Windows starts. That's about how long a "scan all files" takes on a typical 8 - 20 GB drive. If you can find, and open the installed program, you might be able to set it to remove the "scan on boot" instruction, but many demo programs do not allow you to change (very many) of the options.