Update problem

This is my first time using this forum, but I recently had trouble installing a security update.

I am currently using a macbook 5.2 version 10.6.8 on snow leopard.

Anyway I went to do the norma system update and at the very end it said "instillation failed"
ever since then, many of my applications will not open. (iTunes, SMC fan control, system updater ect.) it comes up with a big long error message. I have tried restarting my computer, and manually installing the update, but that failed again.

I do not have money to spend on this so fixing it for free would be awesome!
Any suggestions welcome.
Thanks!

(picture attached is error message from itunes)

I suspect the issue is the first line in the screen shot you posted: incompatible with 10.6.8
Look it up on Apple's site.

By "normal system update" are you referring to checking for updates from the "Software Update" option of your Apple menu? Doing that will open the Mac App Store update dialog if you're running 10.6.8. If you received an error during the update, it's best to download the combination update for 10.6.8 and then try once more to obtain the security update.

10.6.8 combination update download from Apple: LINK After the combo update installs correctly, go back to software update from your Apple menu and try again. The security update may have been integrated into the combo update which will mean there will be no updates available.

Thanks, I will try the combination update. I am away from my mac at the moment but I will let you know the results!

I tried the combination and got the same error as when I first did the security update.
attached is a picture of the message....