Can't access windws shares on network

Hi, I'm new to Ubuntu, and thought I would use 11.04 on my older computer to stream videos over my home network. Everything is running fine, except for access to the Windows workgroup on my network. I am accessing this through the GUI, and I can get to my shared files about 15% of the time. Every other attempt gives an error:

Could not display "smb://renchy-pc/shared folder/". Error: Invalid argument
Please select another viewer and try again.

for a folder I have already mounted, or:

Unable to mount location
Failed to mount Windows share

for a folder I'm trying to mount.
The windows pc, running win7 x64, is sitting idle, with no screensaver or sleep settings turned on, and I have everything set up to share properly on the windows side ( I can access my files 100% of the time from other windows computers on the network).

I really like the new found speed my computer has with this clean install of ubuntu, but if i can't fix this intermittent problem I'm going to have to go back to windows :-(