Actiontec M1424WR Wireless Router - Setting Up Homegroup

I recently moved in with a friend, and we're having trouble setting up file sharing. His PC is connected directly (via ethernet cable) to an Actiontec M1424WR wireless router, which is broadcasting a WEP-secured wireless network. My PC is connected wirelessly (via an RNX-N180UBE usb adapter). Both machines are running Windows 7. Internet access is great. We tried to set up a homegroup, however, and are getting nothing but problems.He created a homegroup on his PC, gave me the password, and I joined on my PC. It said it joined and asked me what I wanted to share, but that's as far as we can get.

- On his PC, when he goes to the Homegroup page, it says "no other Homegroup computers are available."
- On my PC's Homegroup page, I see his computer name listed, and when I open it it shows libraries, but when I double click them it nothing happens
- He right-clicked his "downloads" folder and did "share with -> Homegroup;" I see it in my PC's Homegroup page but when I try to open it, the system thinks for a while before popping up a "cannot access" message
- I also shared a folder of mine to the Homegroup but, since he can't even see my PC, he can't see the folder
- I ran IPCONFIG on both machines; his machine's IP is 192.168.1.2; my machine's IP is 192.168.1.5
- When I ping his machine from my machine, I get "request timed out"
- When I ping my machine from his machine, I get replies.
- When I ping the router from my machine, I get replies.
- He has a printer. On my Homegroup page, it tells me that it has detected a new printer, but the "Install Now" button does nothing.

I've gone into Network and Sharing Center -> Advanced Sharing Settings and enabled "Allow Windows to manage homegroup connections." I did this on both machines.He runs a VPN on his machine. Thought that might be the problem, so we turned it off but nothing changed. I run both Hamachi and Tunngle on my machine. Disabled both of those but nothing changed.