I have got a old work Mac G5 from my work as a private computer. I have formated the harddisk and installer Mac os x 10.4.2. When I did this the Mac told mé that I was downgrade the Mac from a higher Mac os x version. After I have downgraded the Mac I found a new software in the box from my employé. It is mac os x version 10.6.4. When I try to upgrade now the Mac tells mé 'you Can not use the program 'install Mac os x' with this version of Mac os x' I have aldo tryed to start up pushing C without any luck. Please tell mé how to upgrade. Both discs are Grey. I am pretty new ón Mac so give a detail description thanks