I would like to do a clean instal on my XP machine as I have had nothing but problems since I bought it with Vista Home Premium pre installed . 

The cheapest option is to upgrade.  I have run the Upgrade Advisor and been advised that I can upgrade to Windows 7 Home Premium or Ultimate.  My husband is a mature student so I can get an education licence on Ultimate at £57.99.  There is no education licence of Windows & Home Premium available so the cheapest is £98.  That said, Ultimate is way over the top for my needs. 

Am I storing up problems for the future by installing Ultimate, as in when another version of Windows or whatever MS calls it comes out, am I always going to have to instal (and pay for) the premium version?

I have considered biting the bullet now and buying the full version as I'm not sure of the complexity of performing a clean install with upgrades discs.  Any thoughts anyone?