deteriorated signal with two wireless devices connected

Got a special deal for a Dlink wireless router and a USB wifi NIC combo.
The new wireless router is exactly the same as my previous wireless
router, which had been getting flakey, so I replaced it. I've usually
had no more than one device connected to the router through the wireless
connection: my laptop. With the deal, I got a USB NIC too, so I decided
to connect that to a desktop PC that had previously been connected via a
long Ethernet cable. I figured this was a way to eliminate the long cable.

I had been running this setup for a few weeks, but I noticed that the
wireless connectivity was being severely deteriorated. The signal
strength was nothing in some parts of the house (even from a room
adjacent to where the router was). And it was the same way both from the
wireless laptop, and from the wireless PC; they were both noticing the
slowdowns. During all of this time, the wired Ethernet connection was
not affected by it, so it wasn't just a general router issue.

So I unplugged the USB NIC and reattached the long Ethernet cable, and
things are back to normal now; back to a single wirelessly-connected
device again. It's a little disappointing not being able to use the
wireless NIC, but at least there was an alternative. Is it possible that
the router and wireless NIC were creating interferance with each other,
despite the fact that they were supposed to be connecting to each other?

Yousuf Khan