Hi,
I am confused as to what you mean by 'trial version? There isn't a trial version of PaintRibbon, nor will there ever be. At the moment, there is simply PaintRibbon for download - totally free. In the future, there will be an option to upgrade to PaintRibbon Complete, but that won't be available for a month or two.
You should be able to download PaintRibbon, and remove it without any problems! I would advise you try and restart your computer, then delete the file. Remember, PaintRibbon doesn't require installation, which mean you don't have to uninstall it, just delete the file.