Can't access USB Hard Drive

I have an XP home desktop pc wired to a wireless router. This pc is
connected to a USB hard drive and a USB Printer. I have set up a
wireless connection with file and printer sharing to a Vista Home laptop.
The Vista laptop can see all the shared drives and folders on the XP pc
including the USB hard drive. The Vista laptop can access the files on
the internal drives of the XP pc, and can share the printer. However if
I try to access the USB drive or any of its folders (including an empty
folder) I get an error message saying "insufficient storage space on the
server to process the command". The PC has 1.5GB RAM and ample unused
disc space.
Can anyone throw any light on this error? I want to use the USB drive to
store Acronis images of the Laptop, but without having to connect
directly through USB.

Many thanks

Stephen