BUMPFF
I used to get this on my old W98 machine (Compaq, 400Mhz). The way I fixed it until the next time was to take the case off, remove the cable from the motherboard to the HDD, put it back on exactly the same way, and hey presto the machine booted up again immediately.
I know this is a bit of an unsatisfactory fix, but if its the same problem as mine, this could help if you need to get the machine booted up quickly.
cur