Connecting router to existing account

[jackfrost5555]

Hi all,
New here. New to broadband. Just set up dsl with AT@T. No wireless here.
Manual install over the phone. Did not use or want the stuff on the disk. So desktop Dell (XP Pro) with supplied Motorola modem.
Performance can only be described as awsome. Love it.
Here's my problem. A friend told me to buy a router before performing the install. I did. It's a Linksys 4 port BEFSR41 (not wireless). Purpose was to add a second computer after all systems were running properly and the DSL performed properly. So an over the phone installation was performed without attempting to wire in the router.
Well, my friend says, if I used My user name/email/password when doing the setup, installing the the router later, could be an issue.
I saw a clip on the web by Linksys that says just power down, power up and green light the modem, same with the router, followed by the computer, and just go online.
Sure, and Monkeys have 3 tails.
This is a fine system. I don't want to damage any equipment or screw up settings.
I know quite a bit about computers, and dial up settings. But this stuff is all new to me.
Hope some of you people can give me some valuable input. I sure would appreciate it.
Thanks in advance..
Jack Hall