Can't Install Programs Under Vista

It doesn't matter how I try, none of the programs I try to install well finish the install.
I've tried to install AVG free, it goes through all the motions and then just sits there when it get's to the installation bar.
Microsoft office XP, same thing.
The software for my xerox scanner, same thing, I've run them all as administrator, but for some reason none will install...
Any hints?

 

As TimW says, Vista might have a little block on a program, but normally it asks you - if it does not, I found that if you wait, it comes up with a little box that says 'do you want to run this in compatabilty mode' (scanner, possibly), and you carry on from there. If you see nothing, hover the mouse over the bottom toolbar to the right and you will see a little pop up to say programs not yet approved, and you may find them there, awaiting your o.k.