OK then, a few other questions and then some diagnostics:
Since you know what encryption you're using, I assume you also know your wireless key and have entered it in the wireless configuration for each of the three computers. Is that correct?
Also, can you access the router setup page wirelessly?
In other words, if you have a computer connected ONLY through wireless, can you enter 192.168.0.1 in a browser and access the setup page that way?
If you can, you may need to change some configurations on the router or in your internet settings.
If you can't access the setup page wirelessly, I presume you can access it through ethernet?
Either way, we can do a few things to figure out what's breaking.
On one of the two XP laptops, start by making sure it is ONLY connected wirelessly--no ethernet.
Now click start-->run and enter "cmd" in the dialogue box (no quotes). Then click ok.
Once the command prompt appears, type "ipconfig" (no quotes again--the same holds for all other typed commands)--and press return.
You should see a few internet adapters listed, probably including your LAN (probably listed as Ethernet adapter Local Area Connection) and wireless connections. Some (but maybe not all) should have an IP address, Subnet Mask, and/or Default Gateway listed.
Take down that info and post it here--especially the LAN and Wireless information.
Avoid posting any personally identifiable information, such as your DNS suffix or host name. (The first usually identifies your ISP, and the second identifies the name of your computer.)
Now type the following in the command prompt:ping 127.0.0.1
Press return. That's bound to work fine.
Now type:ping 192.168.0.1
and press return.
Finally, ping something on the web--for example,
Note that many websites are not pingable. (microsoft.com, for example, won't work)
In each case, note whether the ping times out or returns a response.
Now repeat the ipconfig routine, this time with the ethernet connected. Also post THOSE ipconfig results.