If you can't Uninstall S10 via ADD/REM PROGs, insert Prog Disk 1 & choose Uninstall.. If that doesn't work, Run RegDelete for S10 & then Delete all Folders/Files for S10, including Hidden Files & Folders.
BTW - If you're going to Uninstall S10 before Installing S12, you won't be able to Transfer purchased Premium Content from S10 to S12, via S12 Transfer Wizard. You would have to Transfer from Content DVD instead.
Either Finish all S10 Projects or Render them to File / DV-AVI, before Installing S12.
I will not be Uninstalling S10 either before or after I eventually Install S12. I have no outstanding Projects & have XP. I suggest you do the same.
ADDED - Read Jeff's Post here: http://forums.pinnaclesys.com/forums/permalink/200124/200132/ShowThread.aspx#200132
In other words - Don't Uninstall S10.