Seeing XP External HDD from Vista Computer via Network

I have introduced 2 Vista computers into the company network. One a desktop, other a laptop. After much frustration, the Desktop can see and exchange files to “shared/public” folders. While not entirely sure how I did that, parallel actions on Vista laptop not successful. Issue at hand: An external HDD plugged into XP computer has installation files needed by both Vista to be compliant with office work and thus, I cannot install those on the Vista computers. I need to be able to see both XP & external HDD. Can you help?