Can't open files from external HD

I installed a new external HD E: connected to my Desktop XP Home SP3. I am wirelessly networked to laptop Vista Home Premium. Sharing is on and network is operating fine.

Problem:

I can save and retrieve files originated on DT to E:
When I try to open a file on my laptop I get the message: 'Can't open read only document'

If the file is in a folder the message is: 'You do not have permission to access the file "\\mycomputer\E:\filename" '
"Ask the server administrator to give you permission to access the file"

I am the administrator on both computers. How can I access these file from my laptop?

Thanks