my mac doesnt want to update

Running 10.5.8 it is up to date.

Note, there is a difference between update and upgrade:

Software update, upgrade--what's the difference? - http://support.apple.com/kb/HT1444

OS X: Updating OS X and Mac App Store apps - http://support.apple.com/kb/HT1338
Reference releases; update vs. upgrade

Mac OS X v10.1, 10.2, 10.3, 10.4, 10.5, 10.6 and OS X Lion themselves are not free updates, they are reference releases, also called upgrades.

If you're trying to upgrade the OS, you need to buy it on DVD instead of using Software Update.

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You need to buy a Mac OS X 10.6 DVD. See above for the link.

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