Windows 7: 3-Way dead end...

Hi guys. Wi-Fi problem.
I have a Linksys WRT54G2 Router, and a Fujitsu Siemens laptop with NO WLAN CARD (yes, it's pretty old - connects with a TP-LINK TL-WN821N USB Adapter)



Now here's the deal:
Whenever the Laptop is turned on and the adapter is plugged, it displays Wi-Fi networks accordingly, but the weird thing is that if I try to connect to my home network it says "Cannot connect to DarkNET". Tried troubleshooting, tried network & sharing center... Nothing. The only possible solution is to go to the router, (192.168.1.1) log in, and then just change some wireless setting to something else. For example the wireless channel frequency (It's 2.462 by default):


Once I do that and get out of the router, the other side gets the internet PERFECTLY - really fast and with no disconnections, interruptions or whatever. Biggest misery is that my dad has an HP laptop (a bit newer; 2x core AMD, WLAN card - built in) and also connects to the same network but has NO PROBLEMS AT ALL! He's on XP Media Center Edition with SP3 and that's where it gets tricky. There is no way of understanding whether it's the router that's lagging up (although I don't think so), my mom's edition of 7 or that TP-LINK USB thing.

Most likely for it to be the USB device. My previous router was also by the TP-LINK brand and it died real fast (lasted only 9 months, dunno why), so it is MOST likely for this stupid TP-LINK thing to be the problem... I've already upgraded router firmware and the problem is still here.

Suggestions???

I suggest getting a Linksys or D-Link Wireless Stick

Umm... any other solutions before I directly go buy something? Plus, I'm a little low on $$$ right now...