No need to apply any crack or hack it is plain simple good old Wine software. It allows to validate an Ubuntu installation as a genuine copy of Microsoft Windows and get to the download page of Windows Defender, using IE4Linux and Wine.
From the Ubuntu forums thread:
I just did this for a laugh and didn't expect it to work. I went to microsoft.com using IE4Linux ( http://www.tatanka.com.br/ies4linux/page/Main_Page ) and tried to download Windows Defender, on Xubuntu. Of course, I was asked to do the Genuine Advantage test. I used the alternate authentication method (IE4Linux doesn't support Active X), downloaded and then ran the genuine advantage application (which took quite a while to start). It gave me a code that I pasted into the authentication box, and to my surprise it verified me and forwarded me to the download page. This just goes to show how rubbish they're validation software is