The Arduino IDE does not require .NET
The Arduino IDE is not really a software installation. It is just a zip archive of all the software.
You just download the software and unzip/extract it to a location on your system. If you are unable to unzip/extract, then that is usually an indication the zip archive was corrupted during the download.
This is the link you should be using: http://arduino.googlecode.com/files/arduino-1.0.4-windows.zip