I'm already at 9.1.008 and am having the same problem. Scan runs to 49% complete, freezes on a .dll file and the hard drive keeps thrashing. I've let it run up to 5 hours and nothing seems to get it going again.
Windows XP SP3 with all updates as of today.
Dell Inspiron 8500 Intel Pentium 4 2.8 Mhz processor.
ZoneAlarm Internet Security Suite.