Most say you shouldn't have a problem... but I recall in the past when I used to use Vista... starting up a computer with two windows installed caused my computer to crash and damaged the bootloader. I've never experienced this problem using Windows 7... but then again I always just wipe the drive first.
If you want to play it safe... and if there isn't anything important on the drive... just boot up your Windows DVD and get the screen where you install Windows... select Advanced Options... and choose to delete partition and format that second drive you added... that will take care of any potential problem and clean the drive... then just exit out of there and boot up as your normally do.