reinstall a program from time machine

So I'm looking to clean up my system and upgrade to Snow Leopard but I have a few applications that I do not have an install disk for. I have a complete backup via time machine on an external, so can and how do I reinstall those apps from my external. And would I need to go into the apps and write down the serial numbers before I upgrade? Thanks in advance for any help

If you have the original DMG files for the apps, restoring them is easy. I have many applications that I purchased on line, some that use a product ID or SN which I keep archived on DVD media in case I have to reinstall.

I mention that because reinstalling an app from a Time Machine backup especially to a different operating system may not work. It's much easier to just do a complete restore from Time Machine when you install Snow Leopard. No system files will be overwritten. I did exactly that when I upgraded from Leopard to Snow Leopard.