Win98 pc frequent fragmentation

I’m trying to troubleshoot a Win98 pc that becomes fragmented to the point that Windows will start only in safe mode until Defrag is run. This happens every 10-14 days, and defrag runs for 3 hours or so. The pc has plenty of available hard drive space. After running defrag, I test restarting several times and no problems, however, 10 days later, the user calls with the recurring problem. Thanks….