Airport Express works fine with iBook but isn't seen by Dell laptop

Question

I have an Airport Express connected to the internet which is automatically recognized by my iBook & works fine. My roomate has a dell laptop running Windows XP and it doesn't recognize the network. In other words usually when the wireless card recognizes a network it pops up in the taskbar and it isn't with the airport. Any suggestions would be appreciated.
iBook   Mac OS X (10.3.9)  

Answer 1

At first we could plug the Dell in directly but that was before setting up the AX. Now that the AX works the Dell cannnot connect even wired. (It says that it's connected but it cannot find server when you try to open a webpage). Could this be a problem that the moden cannot go between Mac & windows?
Most modems tend to "remember" the last device connected to it, so it's important to do a complete power-down/power-up when switching devices. It's possible that your modem did this when you attempted to connect the PC directly. My purpose of having you connect the PC directly was to verify that it can access the Internet with a wired connection and that it's wireless setting needs to be troubleshot further.
The power recycle process should be something like the following:
- Power-off the modem & AX; If possible, leave the modem off overnight.
- Power-on the modem; Wait at least 30 minutes.
- Power-on the AX; Wait at least 5 minutes.