Wireless internet speed?

What I'm about to say makes no physical sense whatsoever to me, but it's what's historically happened on an anecdotal basis.

I put my desktop under the desk (the irony) with its back against a wall. I use a separate wireless receiver with a USB interface, and it's been noted that plugging the receiver in the front improves the performance compared to the back. I've never tried one of those PCI cards, so I can't be sure how well they perform when the antenna is pretty much covered up in every direction. If you have a similar set-up, then there are a few things you can try, which primarily involve moving the antenna to a more open space.

I'll do a bit of rearranging later today, I can probably put it under the desk if I move some stuff around... I got a USB WiFi adapter to see if that'd make any difference since a friend said that had worked for him (and it was pretty cheap) but it didn't change anything, I still get the same speed as I did without it.

Thanks for your help guys!