Well crap. Get Games Go claim they accept PayPal, yet they won't let me register without a credit card. The reason I use PayPal is because I don't have a damn credit card!
No cheap Total War Medieval II Gold for me then.
I tried it and it worked. Try using the same email addy for paypal and your registration if you didn't the first time.
Finally got it sorted. I went back and tried again, it claimed I was already a user. So I logged in with my email and the password I'd picked. Turns out it was an automatic thing, once I registered it took me to the add credit card page. I didn't realise I had registered, I thought it was the next part of the registration process.
The thing is, when I would try to do anything at the site like add a product to the cart, it would redirect back to the add credit card page. I clicked on the back button on the page itself, it changed it to an add address page. Filled that out, and now everything works.
What a fucking terrible way to set up an account!