How to activate an iPhone 3GS if there's an existing iTunes account on the computer?

My mother recently bought an iPhone 3GS. I already have an iTunes account set up on our computer for my iPod. How can we activate her phone without it syncing with my iTunes account and putting all my music and information on there?

I don't think you can have 2 itunes accounts on one computer

In order to begin using your iPhone, you have to activate it. The activation process is different depending on what model of the iPhone you have and what version of the operating system you're running.
http://ipod.about.com/od/iphonehowtos/ss/activate_iphone.htm