I've been raiding Google like crazy to try to solve this but to no avail. Before he bought Vista Ultimate, we used to transfer files between computers in seconds or minutes.
Now, he's trying to send me 20GB of stuff off his computer which at maximum speed, should take no more than 30 minutes with our router. Unfortunately, even after running it for half an hour my XP machine still bounced between 180-240 minutes remaining.
On both computers, by pressing CTRL+ALT+Delete and going to the Network tab, Windows shows no more than 5% utilization of 300Mbps.

We use a D-Link DIR-655 (the XtremeN router they call it) that is capable of transferring at 300Mbps as it uses 802.11n. We also use the matching DWA-552 network cards that we bought along with the router.
I tried calling D-Link but the first response I got from the rep was "have you tried talking to your computer manufacturer?" She insisted there was nothing wrong with the router and after reading what I have, she was probably right.
At the time I hung up on her though because she didn't even try any troubleshooting steps before telling me I was talking to the wrong people. I did over-the-phone tech support for a while and I know that that means "I don't have the training nor the time to spare on my average handle time to figure out what the problem is."

I've read about Vista's network transfer optimization (the actual name escapes me at the moment) that tends to slow down transfers over wireless connections. That's turned off.
I've read about a hotfix from Microsoft's website that sounded like it would fix the problem so I got him to install it. That didn't help. I can't find the KB article but it's not KB931770 like I've seen suggested on some forums on this issue.

I'm stumped. I keep reading the same things over and over on Google but I haven't seen any solution that has worked. Perhaps some of the gurus around here might have some knowledge that I can't find on Google

It's been almost two weeks and I just wanted to bump this back up.

It seems like it's a pretty common issue from looking around but the solution to it isn't so common...