I need help with NAV2011 install

I downloaded and tried to install nas 2011 just like I did three weeks ago, flawlessly, on my  win xp home 2005.

I tried to do the same on another system with the same OS and the downloader stops and displays a message that the OS in incompatable with the downloader.

 I don't get it. If anyone knows the fix I sure need a heads up since I'll be billed auto online.

 

Thanks to all.