ASPI allows certain programs to communicate with your optical drives. KProbe is one such program that needs ASPI, but some CD-recording programs need it, too.
On my Win98 computers, ASPI 4.7x made it so that certain programs couldn't see my optical drives at all!
4.60 works fine, though, so I stick with 4.60.
Edit: Matt, I don't have any USB 2 or Firewire ports on any of my computers. Just USB 1.1.
Another Edit: I have also found that KProbe seems to freeze like this when I attempt to scan a disc in an optical drive that is attached to a PCI IDE controller (i.e. not the IDE controller integrated into the motherboard).