Sharing files between Windows 8 PC and Windows XP PCs

Hello,

I have been working for days to try to get an older XP desktop to get wirelessly friendly with two other computers, one a Windows 8 laptop, the other a Windows XP laptop. The two laptops can share files just fine, but the desktop cannot. All three are now connected to the same Cisco wireless router. They can all get to the internet through that router. But my main PC is the 8. When I look on it for the other two machines, it can see the XP laptop and exchange files with it (this ability was installed when the 8 was a 7, prior to upgrading it to an 8). BUT when I look on the 8 for the XP desktop, it sees the name of that PC; but when I try to access it, it tells me it cannot find the machine. I have tried pinging the desktop from the 8. Doesn't work. But I can ping the 8 from the desktop just fine. Further, I run Norton 360 on the 8 and it can see all the machines, their IP addresses, etc.

The one thing that is different about the desktop is that it uses a USB device (Trendnet) to access the router, not a native card in the PC. (The desktop was given to me by a friend. I wiped the hard drive and did a clean install of XP Pro on it.)

Please help. I have been fighting this problem for nearly two weeks!

Thanks.

WERTRON