My brand new Dell XPS system has been running flawlessly except its abysmal download speeds. I have tried isolating every variable I could possibly think of but I can't figure out the problem. I've talked to Dell and Belkin without making progress (thought I'd try). Here are the speeds:

Note that most of the time, upload speeds are actually much faster than download speeds (around 4.0 Mb/s which is better than most other devices on the network)

It's not the ISP. The slowdown happens even when transferring files inside the network. Plus every other wireless device gets approximately this:

It's not the wireless router. It's a Lynksis WRT160N v1 with the latest firmware (1.02.2). Plus everything else connected to it has normal speeds.

It's not the browser. Speeds are the same in IE, FF, and when transferring files with Windows between computers.

It's not the wireless adapter. I've tried a Belkin N Wireless USB Adapter (which works fine on another computer) and a Dell Wireless Draft 802.11n WLAN Mini-Card. They have the same slow speeds when connected to the problem computer.

It's not the adapter connection. One adapter used USB and the other is a Mini-Card.

It's not antenna placement. With the same antenna position and the same device, I get different speeds when connected to the problem computer vs a good computer. Plus everything reports the connection speed as at least 11Mbps and good signal strength.

I've tried disabling IPv6 since it sometimes causes weird problems.

I've tried disabling Windows Firewall/anti-virus.

I've ensured the computer has updated drivers for both adapters.

I've ensured that Windows is up to date and so is the BIOS.

For the USB adapter I ensured that that USB port functioned at normal speeds with other USB devices.

What else could it possibly be? I finally received my copy of Windows 7 and will be trying that. I'd rather not install Windows 7 because of a particular program that will stop working so a solution besides that is welcome.

Specs:
Vista x64
Core i7 920
6GB RAM
500GB HD
GTX 260