Apple's website is the best place. Curiously, the "delta" update for 10.5.2 seems to no longer be there, but every other delta update for Leopard is still posted.
There are two kinds of updates: A delta update and a combo update. A delta update only works if you are running the immediately preceding version of OS X. That is, you're running 10.5.1 and wish to update to 10.5.2, so you install the 10.5.2 delta update. These updates are smaller to download and probably a little faster to install.
The combo update will update any version of Leopard to the new version. That is, you could be running 10.5.0 and the combo update to 10.5.8 will work. It will also work if you're running 10.5.1, 10.5.2, or any other version that came after.
Ordinarily I recommend the latest combo update (in this case 10.5.8), but it sounds like that is not working for you.
I suggest trying to update to a slightly later version of Leopard first, then trying the 10.5.8 combo update again.
The 10.5.2 combo update is available here. Install this first and see if it works:
If that installs correctly, then try to reinstall the 10.5.8 combo update (I recommend re-downloading from here) :
The 10.5.8 Update is recommended for all users running Mac OS X Leopard version 10.5 to 10.5.7, and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.
support.apple.com
Let us know how it goes.
As suggested by another poster, you also might have a hard disk problem, though it's unusual for the system to totally freeze up in that case; typically it will seem to hang up, but mouse pointer usually still moves, progress bars will still shimmer (but won't advance any farther), you'll eventually get the spinning beach ball, etc. If the system is totally and immediately locking up with no warning, I suspect it's not a hard drive problem.
Try the installs above, but also post back with a couple more details about what you're seeing when the system freezes up.