I'm not real sure you wouldn't do your friend a favor if you "lost" the disk. (Maybe after you get yours fixed?) I can't be certain, but I've checked the last McAfee I have on the shelf (and note that NO version number appears anywhere on the packaging, in the manuals, or on the disk labels) I had to "unzip" part of the installation files to find a version 4.0.1 - and it's almost 3 years old. (The package actually says "New for Win98/95.") The McAfee site "sort-of" says that the current version is 7.0.
It's hard to tell how old your 4.5 demo might be, but they've apparently made some significant changes - to justify rolling the base ver no from 4 to 7(????).
If the "REM" on that line in Autexec.bat lets you boot okay, you can leave it as is or go back and delete it - suite yourself.
My autoexec.bat has about 38 "legacy lines" in it, all starting with "REM" so it isn't doing a thing on my machine. According to "Peter Norton's Complete Guide to Windows 98" Win98 doesn't need an autoexec.bat file unless you have hardware or software that isn't really Win98 grade. Your Soundblaster and UK keyboard may qualify, if they came pre-Win98, so don't fix anything that ain't broke.
Since McAfee shows in the Control Panel Add/Remove software, that would be the place to "uninstall." The installation apparently didn't "finish," though, so it will try to "complete installation" and isn't going to let you into an auto uninstall until it decides it's done with the install. You'll probably want to do some additional "cleanup," but it will be a lot easier after you know what you've got - assuming you can get a good boot back.