do you not understand what a beta is? it is experimental software, when the test period is finished you will not be able to use the software. you should have installed it on a old pc or as a duel boot with vista. if you want vista you will have to go to the store and buy a new disk.
I'm afraid there is no way to downgrade your system without a reinstall of your original system. Try contacting the supplier of your computer and explain what you have done.
I'm sorry that you have found out the hard way about beta testing. You will not be the only one who has failed to read the installion instructions and does not understand the problems with beta testing.