Ok, So I had an account on these forums...alas changed ISP's and now I cant remember my password after having my computer re-formatted 2 weeks ago.
Anyhow, I notice that a lot of my friends and me included had the good old 'Username/Password already in use' error.
We got this every time, even when we used stupid names (look at mine

)
Anyhow, I seem to have found something that has worked for both my friend and I (obviously)
To register.
Go to the forums
Click on a sub forum (I click registration)
In the forum part, near the top of the page you will see the link to register.
Click that link
From there, scroll down the page and click the 'I agree' box,
Type everything...I mean everything...Do not use autofill
Click the hide email if you want you.
Make sure you enter a last name....for some reason it seems to work.
Then once all the required lines are in click register
All the best, I hope it works.