I had this same problem earlier and thought it was a software problem. Finally after several such re-boots one day the power unit conked out and I had to replace it. Things went fine for two months after that. Now again the reboot has started out to occur. After each such reboot the standard Win 98 screen appears advising me to shut down the machine properly next time and automatically runs scan disk!
I have an Intel PII 266mhz with a 4.3 GB Quantum HDD, a16 MB AGP Card, CDRom drive, 128 MB Ram, and an Intel motherboard with 512 KB Pipelined burst cache. The softwares are all working fine. I am certain this is a hardware problem but just can't ascertain what to do about it! I am scared of losing my data. Would appreciate any advice and help!