I did install an older version of windows xp with sp1. When it does restart, it runs chkdsk and I often get messages about files that have to be truncated because they contain errors. This did not happen in the 3 or 4 days when it was running windows 98. I'm not sure if it's relevant, but when I installed XP, I used the FAT32 file system.
In my original post I mentioned the computer had 256 MB RAM, it actually has 512. I have run memtest86 on it for several hours and it has never detected an error.