You could do either. You will need a copy of Ubuntu, either downloaded or from a Linux mag. The downloaded .iso file will need to be burned to a CD/DVD (a USB stick is an alternative, but the CD/DVD is easier). Boot with this in and follow the prompts. Should take about 20 minutes to install.

If you go for the dual boot option, Ubuntu will find the Windows partition and create a Grub bootloader - choose your OS each time you boot.

I'll have to leave it to the Ubuntu users for more details.

Phil