Need help with a peculiar wireless issue

I’m wondering if anyone here has any insight on this.

I have a D-Link 524 AirPlus G router downstairs. My computer is hooked by ethernet cable into the router. Our other desktop is upstairs, connected by a D-Link PCI wireless adapter.

Just today the upstairs machine began having trouble browsing webpages, and with in-game lag in online games. It connected fine to the router, but web browsing or playing MMOs was laggy. I replaced the PCI wireless adapter with an older one, and it was a bit better but still pretty bad. So I picked up a brand new adapter. Same story. Connects at 54 mbps but has enormous lag either going to other machines on the network or browsing webpages. In games, it has lag spikes that reduced movement to, well, stasis, then clear up, only to repeat.

The laptop that is usually upstairs, and which has a two or three year old wireless adapter, connects at 36 and runs flawlessly, with no browser lag whatsoever. It doesn’t seem to be a router issue, but what the heck is causing the desktop’s problems?

Because I reinstalled the card so many times, perhaps, it lists the wireless connection now as wireless connection 4, though there is only one. I used the Windows home networking wizard to connect to our workgroup, and the connected machines appear but it takes forever for that machine to access the other ones; the other machines can acces it rapidly.

I’m kinda at a loss. Is there any way (or need) to, say, wipe all of the existing network information and start from scratch, so the wireless connection is number 1 again? Could this be something related to the Windows networking stuff? The router works, the other machine connected wirelessly works, and even this machine says it connects at 56 to the router, and I can get good ping times to websites even. But actual browsing or using the connection is terribly slow.

Any ideas?