Quote Originally Posted by si_the_geek
There is another simpler option, you could have just one OS, and create a second User who has very restricted permissions (ie: does not have rights to install software for all users).

You can then allow your "normal" user to read their My Documents folder etc if you want to get the files that you have downloaded.

As XP allows you to switch users easily it could be a nice solution, assuming that the limited account fits your needs.
Way to think outside the box! This sounds exactly like what you are lloking for. Enable the Guest account and restrict it heavily. That is what it is there for!