This is a nit, but you probably are wanting to install OS X 10.6 Snow Leopard? OS X 10.8 is Mountain Lion.
Are you downloading the OS X 10.5.8 combo update from here? You probably know to use Disk Utility to repair permissions before you apply the combo update, and then again after you apply it.
Hi Rick, not that it'll help you with your 10.8 wishes, but try getting it diresctly.
Repair Permissions afterwords, reboot.
1058 Combo Update, 10.5.8...
But if you really want 10.8, then...
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Call Apple Sales...in the US: 1-800-MY-APPLE. Or Support... 1-800-275-2273