I've done such for clients and the hours you rack up is costly. But here's what we do. We not only compare what you noted which is nothing but a clue that something software is different but is a clue that some program, setting or infection has caused this.
This is not an offer to teach computer forensics but to share that you would look at all the running programs (preferably right after a reboot to narrow it down) and see what's different.
I'd also take Grif's advice from http://forums.cnet.com/5208-6122_102-0.html?threadID=328220#2968150 (same advice applies to this and other issues.)
I have 2 Dell Dimension 8300 office computers bought with identical systems. When connecting to the internet one connects at 1500 kbps the other at 500 kbps. I am on wireless internet with a local provider using Motorola Wireless network. The radio/antenna is on a 75-foot tower outside the office that is hardwired thru the wall connecting to a switch with both computers connected into the switch. I have replaced the switch, all cords, and even put a new network card in the slow machine with no improvement. The only difference I can find in the 2 computers is that the fast one shows 69%
RAM is used and the other shows 100% RAM is used. They both have 1 gig of RAM total. Could this just be a low memory problem? Does memory affect the internet speed?