EDIT: Updating my first post with all pertinent information I've uncovered since day 1:
I have 3 PCs running Vista. OfficePC, MobilePC, and WifePC. They're all on the same workgroup and can all share media find with each other. They can also all 'see' each other, so in the network section I can see all PC names from each computer.
But when I click on the PC name it's asking me to log in. Once I log in I can see and access all the shares on that PC. However when copying files the transfer rate is EXTREMELY slow, less than 1 MB per second. I should mention that all PCs have an excellent connection to the internet, so the slowness appears to be a software problem.
Here's what's odd. WifePC and MobilePC can actually access each other fine without having to log in. But when OfficePC tries to access either of those PCs or when either of those PCs try to access Office PC, it wants the login.
I tried a fresh install of Vista on OfficePC (easy enough since I have an image of the normal install), set up everything from scratch and still had the same issues.
I've tried all the 'normal' stuff. Network Discovery, File Sharing, Printer Sharing, and Media Sharing are all on. Public Sharing and Password Protection are off - all PCs have this same configuration. I've tried turning Firewall off just for kicks.
I know now that I can share files between PCs without having to log in because I'm doing it on two other PCs. I don't mind just logging in to access this PC, but I'm thinking the slowness has to be tied to whatever is causing this issue, and the slowness is not something I can live with.