Margaret-- The only worry I have about you doing this is if you did something wrong and you wiped vista (of course by doing this you would have a better operating system installed) Just make sure no files are on the computer that you need. Make sure they are backed up. That way nothing is lost.
Remember you can't break a computer by installing an OS.
I really doubt this will happen because the easier option on the installer does all the work for you. I think you just use a slider to choose how big you want the vista to be. I am not sure since I use the advanced option.
Also. If you managed to use that USB installer program to create the Ubuntu USB installer without accidentally copying it to C drive(some people may actually make that mistake, though the program tries its best to single out the flash drives) I am sure you will be fine
