Flash drive appears as portable media device under control pannel

I have several Sandisk Flash On some of my computers, these work great when
inserted. they appear as drive E and F with F being the drive that files are
actually stored on. Unfortunately I have two computers that have problems
when I insert these drives. they both are windows xp pro systems. The E drive
appears, but the F drive shows up under control panel, Protable Media Devices
and the drive can not be used directly. You can only copy files to/from the F
drive. Files cannot be opened or saved directly. I see from searching the web
that this happens to a lot of users, but I have not found a good fix that
lasts. I also have the same problemon a non SansDisk flash drive so i believe
it is something to do with Windows XP SP2. Any ideas? Thanks.