Download Nmap,if you don't already have it. You can get it here:
http://download.insecure.org/nmap/dist/ ... -setup.exe
If you don't run a Windows OS, you can download the Linux version here:
http://nmap.org/install/inst-linux.html
If you have Mac OS X, download it here:
http://nmap.org/install/inst-macosx.html
OK, now you have Nmap. If you are on Windows Vista, go to the Hard Drive(It's located in the "Computer" Icon, under the label OS). Once you are in the hard drive, go to the folder that's named "Program Files". Find Nmap's folder. Right click on Nmap's Icon and choose the tab that says "Compatibility". select Windows XP from the Dropdown Menu, and select Apply.
Then Click the box that says "Run this program as an administrator".
Now, go to the desktop and try and use Nmap. It's working!
If it's not, just PM me. You'll get a full refund

Enjoy!