Networking & wireless forum: Two wireless computers - internet suddenly slow on one

Basic network setup:

Motorola cable modem ---> Ooma VOIP Hub ---> D-Link WBR-2310 Rangebooster Wireless G Router

Computer A: Dell Vostro 1500 Laptop w/integrated wireless g (Vista)
Computer B: Dell Inspiron 531 Desktop w/integrated wireless g (Vista)

Security: 128-bit WEP Key

Router location: home office on 2nd floor of house, on top of wood cabinet.
Laptop location: all over, but usually in the same room as router.
Desktop location: downstairs kitchen in a computer "nook" on same side of house as router.

The Problem:

Last week, the desktop computer's internet connection started running very, very slow all of a sudden. I have had it for two years and for the most part, it has run fine. I have always been able to download large files quickly and I play FPS games online just fine with it (was just playing Left 4 Dead 2 online with no problems a couple weekends ago). It also wouldn't complete Windows updates starting last week and started freezing up all over the place. So I decided to just wipe it clean and re-image it.

Re-installed Vista this weekend, but even with the now clean system, the internet still runs slowly. I have completed round upon round of Windows updates and have gotten Vista SP 2 installed.

I have installed a limited amount of software on the desktop so far, including Kaspersky IS 2009, Nero 9, Microsoft Office, Firefox, PC Anywhere, and some Canon Utilities. The internet slowdown was happening before any of these were installed, though.

I have noticed that even though in the Network and Sharing Center it says the wireless signal strength is "Excellent," I am rarely getting speeds faster than 11.0 Mbps. In fact, it's often down to 1.0, 2.0, or 5.5 Mbps. In the meantime, my laptop, which as far as I can tell has the same wireless settings, gets 54 Mbps consistently, never falling any lower than 36 Mbps, and that's only for a very short time. I'm getting these speeds when the laptop is next to the desktop, too.

Normally, I'd suspect computer placement as the cause or maybe the placement of an object near it is causing interference (there is a subwoofer on top of it), but considering a) I've had the computer for 2 years (1 year in this specific spot in the house) and never had this problem until last week, and b) the signal strength is still Excellent, I don't think that's it.

Any thoughts as to why my desktop's internet speed is all of a sudden unbearably slow while my laptop is fine?

Thanks!