I tried to install Paralells on my macmini with windows XP. During the installation process i found out i lost the xp reg key. So i bought a new Windows OS, Vista. Now i'm trying to install the Vista software, but when starting paralells desktop again i immediately go to the old interrupted XP install screen. I already removed parallels fully and re-installed, but nothing helped !
Search for the .hdd and .pvs files, those are the files that you will need to delete to take the old install off your machine.