Slow browsing on desktop, but not on laptop

I have a Linksys BEFSW11S4 ver. 4 router directly connected to my desktop and I connect with my laptop wirelessly. My problem is that the browsing speeds on my desktop are very slow. Webpages will take 15-30 seconds to load, while the exact same site will load instantly on my laptop as normal. Oddly though, when I run a bandwidth speed test I will get the same good results with the desktop as I will with the laptop.

However, certain speed tests will time out and not even start. This timing out seems apparent on the internet, maybe... because sometimes the page won't do anything for 15-20 seconds, and then pretty quickly load up all of a sudden. Again, this is all completely on my desktop only, the wirelessly connected laptop is completely normal. Also, I don't know if it means anything, but when I cycle my DSL modem and restart, sometimes the internet will go normal speed for about a minute... then back down to slow. I'm not sure if this is a malware issue, but I went through an extensive Windows XP Cleaning process from a guide on the MajorGeek's Malware Removal forums, and I'm still having problems. http://forums.majorgeeks.com/showthread.php?t=139313

Some more information to add... my desktop is Windows XP, and my laptop is Windows Vista. Both have the most recent updates. Also, I installed a USB wireless adapter for my desktop, and is HAS IMPROVED the situation... but it's still slower than the laptop. This initially lead me to believe it was actually my desktop's network card that was faulty... but since I still have the problem (although to a lesser extent) I'm still not sure. Thanks for any help!!

 

Can you rule out hardware faults -
Try a different port on the router.
Try a different ethernet cable.
Can you put the network card in a different slot on the motherboard?
Tried updating drivers for network card?
Does the same thing happen if you boot into Safe Mode with Networking? Hit F8 at Windows boot up.