I would like to update from 10.5 to 10.6. I have a Mac Pro.
Model Name: Mac Pro
Model Identifier: MacPro2,1
Processor Name: Quad-Core Intel Xeon
Processor Speed: 3 GHz
Number Of Processors: 2
Total Number Of Cores: 8
L2 Cache (per processor): 8 MB
Memory: 4 GB
Bus Speed: 1.33 GHz
Boot ROM Version: MP21.007F.B06
SMC Version (system): 1.15f3
System Version: Mac OS X 10.5.8 (9L31a)
Kernel Version: Darwin 9.8.0
Boot Volume: Macintosh HD
Boot Mode: Normal
To upgrade from OS X 10.5.8, you must first purchase and install Snow Leopard on DVD.
Elsewhere: call the phone number in the Apple Online Store
Apple's price is $19.99, £14.00, €18.
Confirm that you booted from the Snow Leopard Install DVD and that it is the retail version purchased directly from Apple.
If that does not work:
Boot your Mac Pro normally, then download and install the Mac OS X 10.5.8 Combo update.
Reboot with the Snow Leopard Install DVD and try again.
Reset the NVRAM and try again.
If that still does not work, be certain that you have archived your existing system using Time Machine or suitable "clone" software before proceeding, because it may be necessary to restore your system using your original grey System Install DVD.
Incompatible hardware is a possibility. This could include aftermarket memory, non-original video cards for which no Snow Leopard driver exists on the DVD, a corrupt and unrepairable hard disk, or something else broken in the Mac Pro.
The Snow Leopard installation disk you are using looks like this?
It must be white as shown in the picture, and not have "OEM" or "Upgrade" printed on it.
Snow Leopard performs a number of tests before installing on your Mac, and one or more are disqualifying your system for reasons unknown.
To reinstall your original OS you may be able to purchase replacement discs from Apple if you call them and provide your Mac Pro's serial number. To find it select > About This Mac, and click the grey "Version 10.5.8" text twice.
When you receive the replacement boot from the System Install DVD and repair your Mac Pro's disk using Disk Utility. The discs will also be necessary to run Apple Hardware Test, which might reveal what is preventing Snow Leopard from installing. It could be something as simple as faulty RAM.
The only way to revert to your Mac Pro's original OS is to completely erase the disk. This is the reason you must first back up your system, unless you do not care about the data you already have.