Have you downloaded/installed/used Kazaa ?  If so, you may have gotten Altnet from there.

Here is a short synopsis:

"File-swappers using Kazaa were unwittingly downloading software that could turn their computers into part of a new network.  Kazaa quietly has been bundled for two months with software that contains the core of a new peer-to-peer network. This software, from a California company called Brilliant Digital Entertainment, has been installed on potentially tens of millions of computers....  Brilliant Digital created a new subsidiary, called Altnet."

 

You can find detailed information, including technical/manual removal instructions, at the following site (Just read this, do *NOT* use their 'detection tool' --- Just hit the 'X' to close down Scan Spyware):

http://www.scanspyware.net/info/Altnet.htm

Warning:  1) unless you are highly experienced, do NOT attempt to edit/modify your registry.

                  2) I am presenting you this article "AS IS".... meaning I cannot vouch for its accuracy.  If you choose to follow it, you're doing so on your own.  I cannot take any responsibility for your actions here.

 

Perhaps someone else, who has actually experienced and recovered from this problem, will be able to offer you first-hand advice.