What were your previous partitions set up as? Do you have /, /swap, /home
or did you set something else up?
if you have a seperate / and /home you can format your / partition and install Mint there, leaving your swap and /home the same. That should also preserve your files in your /home directory.
*edit to say-
If you don't have anything of particular value on your /home partition you can just install Mint using the cd and have it install using the whole harddrive just like ubuntu. That will format your harddrive, erasing your whole harddrive, and giving you a clean Mint install. I don't know your experience level, but if you don't have anything you need to preserve this option would probably be easiest. More information on how you want to set up your computer will give more specific options as to what you can do.
"If it's stupid and it works.... it's not stupid"