Problem solved... that was not an easy one and I'm sure this will happen sooner or later to someone else so here is the fix.

Network card is an Atheros L1 and when I installed Windows 7 it had the lastest driver so that was great. After a few days I experimented the problem with upload only being interrupted and had to reset the NIC and all was fine. It was happening with WinSCP but also Gmail attachment so I knew it wasn't only related to WinSCP.

To solve the problem, go in NIC properties and turn OFF the Task Offload.

Voilà!

On XP I never had a problem but now that I'm on Windows 7, when I upload file over 65535 bytes, WinSCP stop responding and my internet connection break. I need to do a repair using the icon at the bottom to get access to internet again.

I was using 4.24 and tried with 4.27, same problem. I enabled Passive mode, same problem.

I'm lost... I have no idea why this is happening.