Question : Problems with one machine on a network

Hi, I have 3 computers hooked up to a small home network to share a DSL connection. 2 of the computers (coincidentally both older win98 machines) have no problems whatsoever. The one with the problem is a much newer machine with WinXP. I don't know much about networking, but here's my description of the problem. It appears that I am getting extremely varied download speeds intermittently. The most clear example of this problem is when I am opening a website with flash that displays a progress meter. The progress meter will initially zoom up very quickly (indicating fast downloading at the speed I am used to) then all the sudden stop and chug along for a few percent (indicating slow or zero speed) then resume downloading at a fast speed then back down to chugging along etc etc. Another good example is opening high res pictures with a large file size. It will quickly download a bunch of the top of the picture then slowly download a few rows of pixels then resume with fast speeds again. Sometimes however the picture will stop downloading completely with the browser showing "Done" in the status bar, even though the file is clearly not downloaded completely. Also I believe I have noticed that large downloads (10+ MB) will sometimes simply get cut off with no explanation, though this is uncommon. The entire cycle of going from fast to slow and back again takes anywhere from 5 to 15 seconds. This flunctation of speeds does NOT average out to be as fast as the download speed of one the other computers on the network. I know my description of the problem isn't the best, but I will be more than willing to provide additional info.

Other notes/Things I have tried:
1) The computer in question is less than a year old with WinXP Home SP2. It is fully up to date. No viruses/spyware are present. It uses the onboard ethernet on the mobo.
2) I have no firewalls or other network monitoring software installed, other than the Windows firewall.
3) This problem has only been occurring for about a month. The onset of the problem does not correspond to any newly installed programs or updates, nor any hardware changes. The setup of the network itself has not been changed in nearly a year and has been problem free until now.
4) I have switched out ethernet cables and tried plugging the computer into a different port on the hub.
5) I don't appear have any other problems with the network. I play games online and use an instant messenging client (both of which require a constant connection) and I never get disconnected from them. I frequently use Windows Remote Desktop Connection as well with no problems.
6) This problem occurs in both IE and Firefox.
7) I used an online bandwidth test on all machines on the network. I know these things are not completely reliable, but it consistently shows the bad computer at 600kb/s while the two good computers consistently get close to 1500kb/s.

At first I thought I was just imagining this problem, but it is fully reproducible. I can view a large photo online within 5 seconds on one of the good computers and then move to the bad computer 10 feet away and watch it download the same file over the course of 20 or more seconds. Needless to say, this is extremely annoying. Parially download pictures and dropped downloads make it even more so. Any help would be greatly appreciated! Thanks.

Answer : Problems with one machine on a network

I don't think I've ever seen a Windows XP machine that has an MTU set to anything but 1500.  Not to mention the machine worked fine before.   dslreports.com has a "Tweek Test" in the tools section that will tell you about your TCP settings.