Windows 98 SE

Windows 98 SE makes a relatively nice QEMU guest because it's lightweight.
The downside at this point is that getting support from Microsoft for patching
and the like, as well as third party software (Mozilla, Real, Apple, Adobe,
Macromedia) is getting pretty thin.

Windows 98 SE is a 32-bit'ish OS, so you can run 32-bit kqemu and get
some performance improvments.

There are two drivers you'll want to get

1) realtek 8139 nic driver
2) es1370 audio driver