SBC/Yahoo DSL and Two Dell PCs - Connectivity Probs

I recently signed up with SBC/Yahoo DSL and have connectivity problems. I'm hoping somebody here has some potential solutions.

The PCs:
Dell Dimension 4600, connected via ethernet cable
Dell Inspirion 1150, connect via wireless
Both use XP Pro, Service Pack2

The Dimension has ZA Suite installed. The Inspirion had a trial version of McAfee Security Suite pre-installed, but I've completely deleted it to rule out any firewall issues. I've used the Inspririon in several hotels and T-Mobile hotspots (read "Starbucks") without any problems, even with the firewall installed.

The SBC modem/router is a 2Wire.

Here are the problems:
Wireless - connectivity appears to about three feet. If I go into the very next room and put the computer about 16ft away from the modem, I cannot connect even though "ipconfig" shows an IP address for the modem/router. If the laptop is that the same desk as the modem, connectivity appears to be fine.

Ethernet Cable - When either PC is hooked up via an ethernet cord, connectivity is generally ok. Except, if I leave the desktop unattended for an hour or more, then I cannot connect. I have to restart the Dimension to regain the connection.

Other factors:
*My cordless phone is brand new and is 5.8ghz. No 2.4ghz phones in my house.
*I live in Chicago, which means, there is somebody right below me and houses are very close together.
*I can constanly see other wireless conections, including two that also appear to be SBC/Yahoo DSL. I can't connect to them because of encryption (I'm promted for a username/password).
*I'm using a Nokia CRT monitor for my desktop
*The person who lived here before used Comcast and said he didn't have any probs with his wireless network.

Any suggestions/advice would be appreciated.