One Way File Sharing

My first post on here and it's a biggie. I have tried everything I can think of to no avail.
I have two computers networked one called Upstairs and one Downstairs for obvious reasons. Both computers run XP Pro (SP2) and are connected via a BT Home Hub, which I believe is a Speed Touch router by cable (no wireless connections). I am the only user on Upstairs and Downstairs is used by myself as administrator and my daughter with a Limited account.

The problem is that I cannot access any shared files from Downstairs. The folders show up OK but when I try to access them, I get the standard message. "\\Upstairs\filename is not accessible. (The exception to this is the two bits of music in the Shared Documents/Sample music section. A bit of Beethoven's 9th and some blues track. If I put any of my music in there though - No good)You might not have permission to access this network resource. Contact the administrator of this server to find out if you have access permissions. Access is denied" Both computers access the Internet fine and I have no problem accessing files on Downstairs from Upstairs. I have tried to do it with no firewall on either computer - same result. I have run several reputable malware removal tools over both computers with no significant problems found. Have run the network set-up wizard countless times. Made sure all the recommended settings were in place. If I were to try and list all the stuff I'd done, I'd be here till midnight. I have only had the BT Home Hub since July and the problem was occurring long before then. I was just using a Voyager modem, with Downstairs connected directly to Upstairs via the Ethernet cable. The problem is less acute that it was, as I've just reloaded Windows on Downstairs and now it will "see" my external drive (It wouldn't before). I can move files around using that.
Hope somebody can help me with this one. I have a massive backlog of stuff now which I need to keep and I'm rapidly running out of space on my hard drives and need to test a lot of my work in both directions. Costing me a fortune in DVDs.