Description:
Since recently I've been having a strange problem that affects all xp computers I have. The problem is that internet connection keeps resetting for everything that takes time to dl. Which means that I can browse pages without a problem but if I try to dl something it starts but after several seconds the connection is reset. Or browser starts to dl an image in a page and if it is big enough (0.5M) I can see the part of the image been downloaded and then it just stops leaving a part of the image shown in IE or nothing in FF. Videos on youtube start to play but after several seconds the progress bar shows that video has been downloaded completely although only a few seconds of the video is actually downloaded. Some pages are showing - 'Connection to the server was reset while the page was loading'

Facts:
- I'm using xp standard firewall and Ms security essential. It doesn't matter if firewall is active or not - the problem persist.
- Ms security essential is up to date and finds no malware
- I've also tried gmer (whatever it is) and it detects nothing
- It doesn't matter if I use IE or FF
- I've already tried to reset winsock and tcp ip using netsh - doesn't help
- A wired XP and a wireless XP computers do have the same problem.
- The problem have started to appear on two different xp computers at the same time
- I've removed Google Updater - didn't help
- I've checked eventvwr - there's nothing suspicious there
- A wireless linux computer is not affected and works fine!
- googling for 'connection to the server was reset...' didn't really help
- sysinternals rootkit revealer detects nothing

I could think of:
- a router problem - working linux computer shows that it is not the router problem.
- a problem that came with the recent windows update - one xp computer was never updated after upgrading to sp3 and worked fine until recently.
- a firewall problem - both xp computers worked well with the same firewall settings until recently.
- a network driver problem - one xp computer is wireless another one is wired so they have different network drivers.
- a very well hidden malware that has infected both xp machines

I really do not want to reinstall windows because I need to know the root cause. Otherwise it'll be necessary to reinstall windows every time I have this problem.

So if anyone has any idea or suggestion it would be great.