I’ve come across KernelEx – it’s an open source compatibility layer for Windows 98 and Me which allows programs designed for Windows 2000 and XP to run on the older operating systems. I came by it on the VLC forums, where there are screenshots of VLC 1.0.1 and Firefox 3.5.2 running even though these programs normally wouldn’t run on such an old copy of Windows.
I can’t test KernelEx because I don’t have a copy of Windows 98 or Me to hand. In any case, both operating systems have been long abandoned by Microsoft and are probably full of unpatched security holes now. But if you’re feeling nostalgic, or just like the geeky satisfaction of getting something to work that shouldn’t normally work, give it a shot.