What percentage of corporate PCs are locked down so that a user
can not install any additional software on that computer?

If possible please break this down into software that can be installed
by someone with administrator rights on the computer versus a local
user.

Please indicate source and links.

It was always a bit difficult to come up with a balance between a
machine that was 'locked down' vs 'keeping the machine fully usable.

For example, sales reps are notorious for loading all sorts of
dangerous programs. But they are the most likely to need installation
rights while on the road, for example to view a customers demo CD that
maybe comes with a reader that the sales reps need to install.

So a better method is to scare the pants off them by asking them to
sign an agreement not to missuse company equipment, with well formed
'Can & Can nots' so that the line between what they can load and not
load is very clear.

Those documents are notoriously difficult to word, but they are some
established paragraphs that tend to be used. How good they are has yet
to be tested in court as far as I know.

But it is infinately better than the lock down method, because no
functionality of the PC is lost for the sake of the rogue few.