Seemingly Inexplicable Wireless Stability Difficulties

About 6 months ago I helped a friend of mine build a computer. Seeing as we chose 8 GB of RAM for it, she opted to install Windows 7 64 bit on it. She literally transferred the PCI wireless card from her old PC to the new one, but it simply wouldn't stay connected to the router reliably (despite the signal being strong), so she bought a new one. That didn't work either. Eventually she gave up and opted for a wired connection.

She moved, and now she's in a position where a wired connection is very inconvenient. The same problem is back again with a completely new wireless network. I decided to send her a USB dongle because we were concerned that the PCI Wifi card problem was caused by a motherboard issue (it was the only good explanation that anyone suggested to us). Now it's not working very well with the USB dongle (but it appears to be working somewhat better, I guess).

To sum up, we've having wifi problems and:

1) It isn't a signal strength issue.
2) We've tried 2 different PCI cards, 1 USB dongle, and 2 routers.
3) Various drivers have been changed but no fresh installs have been attempted.
4) I don't believe it's configuration based, but it could be something extremely obscure.

Right now I'm thinking it's one of these issues:

1) The motherboard is messed up in a way that interferes with both the PCI cards and the USB dongle. This is kind of a baseless guess because everything else on her PC works just fine.
2) Her install of Windows 7 64 bit is somehow bad.

I'd appreciate any thoughts, we've tried most basic things and I read over the FAQ and did a lot of googling first. This has been an ongoing problem for months.

A bit of additional info from her:

[18:13] Sarah: I dunno how it could be the mobo, seeing as all other USB devices work properly
[18:13] Sarah: And my video card isn't seeing any trouble
[18:13] Sarah: If it matters, I've tried all PCI slots, as well as the PCI-E