I had the same problem with Windows ME. Changed the MB on an old PII for a seemingly identical MB, just with a different BIOS chip. Seem to remember the old one had Award Bios and I didn't like it so put in an Ami Bios one instead. Anyway, Win Me fell over in a heap and won't boot. Prehaps Windows in general can't handle a BIOS change.
Strangely enough I did the same MB swap of the same boards in a different case a few weeks later and the system booted. Difference this time was that I had Red Hat Fedora 2 loaded, so Avtrician is correct.
Regards
Stoney
Not that we doubted Avtrician in any way! 