A very interesting problem

I'm having a very interesting problem. Yesterday evening I tried to make a post on another forum. I couldn't because the connection was terminated. I thought maybe it's just a temporary problem with the remote server. I tried again today with the same result.

After that I tried to send email via my ISP's mail server. It failed. I tried again using gmail's webmail. It failed. I tried again using my school's webmail and you guessed it, it failed!

Then I tried to upload an image via FTP and it failed. Then I sent a test email message to a friend, and it surprisingly succeeded! After that I tried to send that same message with a picture attached and it failed. Then I tried to upload the image with SCP. It sent 4kB and then failed.

I really needed to upload the pic, so I SCP'd it into the computer running Debian right next to the Windows computer I originally tried with. No errors. Then I SCP'd it forwards from the Debian box, again with no errors.

Shortly put, the error occurs when all of the following conditions are met:
- The data to be sent is larger than 4kB
- I'm sending the data from the computer running Windows XP
- The target machine is outside my LAN

This is not affected by the protocol used or the type of the data sent.

Can you come up with anything? I don't know what I should try. I will call some tech support number next Monday if I just can decide which one to call. Microsoft? My ISP? Telewell?

I was also able to scp a bigger file to my friend's server, which is in the same town as I am, and uses the same ISP.

That it very strange...things to try.

1. reboot..if you haven't already.
2. check your router and make sure there aren't any strange filters applied for the IP address of the XP machine.
3. remove and reinstall the drivers for the network card in the XP box.
4. create an account at dslreports.com and then go into the test and tools section and run the packet loss tester.
5. reapply the latest service pack for XP (you should do this after reinstalling your network drivers anyway).
6. ???