Customize the registration form on membership

Hi,

I would like to add an other field to the registration process for my membership site. Is there a way to do this? At the moment its:

Username
email

Password
Confirm Password

I would like to add a field with a drop down box for Country.

This is because we don't need to pay VAT on members outside the UK so its really important for us to be able to find this out.

Thanks for your help

Paul