Scandisk won’t scan

I have a Dell PIII, 450 mhz, running Win98. My software keeps locking up at odd times. I tried running scandisk and defrag but I get an error message stating that another program is writing to the disk and has to be shut down in order for scandiskto work. I have tried shutting down everything I can find and disabling all the software working in the background that I can find but I still can’t locate the program that is writing to the disk.