How to add links in the title for a profile text field
It sounds like you should be looking at the Terms of Use module, which will place the check box on the user registration page. It provides a link to a node, which you create, outlining the terms the user must agree to to register on the site.

On the project page they also mention the Legal module, which provides similar, but more extensive options.

Check both, and see which best satisfies your needs.