Attachments gets corrupted in several e-mail applications

Windows 7 64 bit

When I send an attachment (that opens perfectly on the computer), the other side gets it corrupted. PDF cannot be opened, even JPG open corrupted.
In my 'sent' folder I can open the attachment.
When I e-mail the attachment to myself, I also cannot open the attachments.

It happened on Windows Live Mail 2012, and also on Outlook 2010.

Even when I try to add an attachment to gmail, it says "attachments failed".

My antivirus is Microsoft Security Essentials, but even when shutting down real-time protection, it still happens.

Any idea?

More information:

Windows update sometimes works and sometimes gives error 80072efe.
I tried using a different router/modem and a different internet connection, same results.
I updated my network card driver, same results.
I ran sfc /scannow, same results.
Antivirus cannot find any virus or rootkits.

Thank you, A Guy!
I tried what you suggested - safe mode, clean startup, but got same results.
It looks like any upload fails - sending attachments, attaching a file to gmail, even wetransfer.com

Thanks A Guy!

I did the Microsoft fix, and it didn't help.

However, when I took the computer to another house, with another cable connection and another modem/router, everything worked!

It seems like the problem is with the upload of the current modem/router/internet connection.
Any idea of any software than can measure errors in the upload stream?

It seems like a noise/connection problem because the files arrive each time with a different error, and if its a picture, the picture arrives each time with a different defect.

I pinged the router for a few minutes, and I saw packet loss of 8%.

Thanks, Nir.

Thanks A Guy!

It is a modem+router (cable Internet service provider), and I cannot test it on another home because the other home doesn't have the same cable company.

I will call the ISP.

Thanks!

Still in progress...
ISP paid a visit, and replaced the modem - nothing changed.
Then I installed a different network card, and some imporvement showed - attachment on gmail worked, but on wetransfer and other web services not. Also, as before, attachments didn't work on Outlook, backup app couldn't upload files, windows update didn't work.

Strangely indeed gmail attachment did work after replacing the network card.

Then I tried booting into a USB puppy linux - and checked gmail attachments as well as wetransfer attachements. Both works.

So, my next try is to do a clean install of Windows.

Wish me luck - this is the stragest PC problem I faced so far...