Hard Drive Stalls & Locks Up

I have a client using Win98 on an 8GB IDE drive. She reports computer locks up whenever serious multitasking is attempted (e.g. minimize one program and open another). CPU is PIII/450Mhz. I’ve noted that Scandisk and Defrag both get approx 75% complete, then cycle into endless restarts. Virus protection has been good, so I don’t think it’s a virus. The Scandisk/Defrag problems lead me to suspect it’s a hard drive problem, but before I sell her a new drive, I thought I’d see if anyone else hasseen a similiar problem. Thanks!