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you can uninstall it with the revo uninstaller which should do a better job
you can use the free one on the left. No need to even get the trial version of the pro one. the free one will work fine for this.
you can also try 2 things when doing the install
1. right click on the setup file and select RUN AS ADMINISTRATOR
2. it should either show you where it is going to install it or give you an option for custom install which will show you where it will install it. Change that folder to another name. you can just add a 2 at the end. all that matters is that it is not the same folder it usually installs into.