Hi,
My guess is that you are using a Windows PC. If so, go to the "Control Panel" and the "Program and Features" scroll thru the list and see if PixInsight is listed. If so, select it, right-click and un-install. You should "reboot" the pc to get a fresh new start before re-installing PixInsight.
If PixInsight is not listed then you might need to run some other software to clean up the "Registry" and remove old files etc. I have used a program called "Revo" to do this.
John