Wireless network woes...help.

I have recently endured countless hours of troubleshooting spontaneous reboots from one of the pcs on my home network and as a result I have pretty much rebuilt the entire computer....motherboard, RAM, hard drive, power supply. The latest "upgrade" was the motherboard.
I'm now trying to reconnect this computer to my wireless network. Here's my setup:
WRT54 Router with my "main" pc connected via cable. It works fine. A notebook which I use occasionally which connects via a Notebook adapter. It also works fine and finally the culprit: A desktop PC that connects via a USB adapter WUSB11 Ver.2.8 . During the troubleshooting process I also tried a new USB adapter...a G standard one, but the reboots continued.
I have just completed putting the motherboard in and am trying to set up my internet connection. The USB adapter installed ok and shows a good connection but I can't connect to the internet. In fact it has rebooted a couple of time when I've refreshed IE in an effort to connect. The only way I can get connected is to disable all security and the router's firewall.
Prior to this when I have had to reinstall, reconfigure, etc. I have simply assigned a network key to the computer and it has connected fine.
Also this is obviously causing at least some of my rebooting problems.
I'm sure this is something I'm overlooking but I would really appreciate some help!
Thanks
Gary

Btw all my pcs are running XP Pro.