I've been dealing with a Dell Dimension 2350 and its being stubborn.
I installed XP Pro. It had Home Edition prior. Since dell sets there startup sequence on the hard drive before the cdrom I went in and changed statup sequence and installed from cdrom. I deleted the previous partitions - one made with 30 mb? and created new one installing XP Pro on that. Now when I bootup a screen comes on before letting into windows asking -
Please select the following operating system to start:
XP Pro XP Home
If I select Home an error message comes up asking me to restart. How Do I Delete the old XP Home Files that are still left?
That will get it rid of it from showing up in your boot options.
As for the files - when you installed did you tell WinXP Pro to use the default directory or did it create a new one? If it created a new one, simply deleting the old directory will solve your problem of wasted file space.
If it used the same directory, then I don't know of a way to get rid of any surplus files.