XP Wireless network

Hey everyone ~ here's one that is stumping me....

My friend's have 3 computers - a brand new Dell XPS, a 1-yr old HP laptop, and an older garage-box desktop.

The two desktops have brand new Linksys wireless G PCI cards installed. Laptop has integrated wireless. Laptop and older desktop are running Windows XP home SP2, with all updates, and Dell XPS has Windows XP media center SP2 with all updates.

They recently got high-speed internet out here on their farm, and wanted a wireless network setup in their house, so I've been setting it up for them. On all three PCs, they've got a profile for each user (4 kids, and their Mom = 5 profiles). I've been using their Mom's profile, and all of them are administrators on each PC.

I have shared the printer from the new XPS. When I go to the older desktop, I can see shared printer over the network, and can install it, but when I try to print, it immediately fails. If I try to browse the shared files on the XPS over the network, it sees the folder but says ACCESS DENIED or FOLDER DOESN'T EXIST. However, everything is there and powered up.

When I go to browse the network from the HP laptop, it tells me that the network is not available, and doesn't let me view anything. However, all 3 computers are on the Internet just fine - just two will not see the network.

From the XPS, I can view all folders from the other two PCs. I can add the printers that are installed on the other two PCs. The only thing that I noticed at first was that the XPS is basically sitting right next to the router. The older desktop was about 15 ft away, and the laptop is about 25 feet away. I moved the router so it was in more of a centralized spot, but that has made no difference in the networking attempts. They all have either "Excellent" or "very good" wireless connections.

Can anyone help?

Thanks!