A very strange behaviour: Internet slows down during surfing in Windows 7

I am desperately seeking some help for a big problem in my PC with Windows 7 cable connected to an ADSL modem. Since a couple of days ago I noticed this very strange behaviour: just after a new system boot, I can surf the web as usual with my browser. But after a couple of visited pages (not more), internet slows down and becomes virtually impossible to use.

The only way to temporarily recover internet use is to reboot PC from scratch (disconnecting as user doesn't work). But even so, after a couple of click, internet becomes unusable.

I made many tests to try to understand more about the problem:

1) I tried with different browsers (Chrome, Internet Explorer, Opera, Firefox) but the progressive slowing down is always present (so it is not a problem of browser);

2) on another PC with Ubuntu connected to the same Digikom ADSL modem-router, internet works fine as usual (so it doesn't depend on my internet connection); I tried also to change cables and connections, with no result;

3) as I have a old image of my system, I reloaded it with Norton Ghost to see if the problem depend on some recently installed software or virus: but after loading previous image, the problem was again there as before;

4) I tried to reset the modem with the reset button, but that too didn't solve the problem (so it doesn't depend on modem);

5) I booted the PC from a Linux Disk and in this way internet is working OK (so it definitely depends on Windows 7).

That's all and I am completely puzzled. Even a little suggestion, some idea, everything would be greatly appreciated.

When your internet slows down can you open up a command prompt, run 'netstat -ano' and copy the results here?

Also can you run 'ipconfig /all' in the command prompt and post that as well?

Do you have any ports forwarded to this PC?

Are you running any server type programs
torrents, ftp, p2p games, etc.

If you change the IP address does the problem go away?

Thanks,
Michael H.

I managed to solve the problem disabling all unnecessary services at startup! Clearly I had some bad piece of software in my system that blocked my surfing.
Thanks for helping!

Not a problem. I am glad you were able to get it fixed.

Respectfully,
Michael H.