I heard from my friend who has a mac that when you have installed some windows software on the windows partition, you are able to run the application using Mac OSX Leopard. Is this true?
Parallels and Fusion can run Windows apps in "coherence mode", meaning side-by-side with OS X apps after a full Parallels/Fusion installation and a full Windows virtual machine installation.
Yes, using VMWare Fusion or Parallels. Alternatively, a limited range of Windows software is supported by Crossover Office, overcoming the need to install Windows at all for some folks.
It's possible with Parallels (practically 100% compatibility with Windows apps, but relatively slow, especially with games) or Cross Over (not particularly compatible, but very fast).