Preventing Parallels from opening

I'm running a Mac Mini with only 2GB and Parallels 6 (with Windows XP) which I don't need most of the time except for very specific work. I also run CrossOver Mac which is lower on RAM for Microsoft Word etc. Occasional, clicking on a file, maybe something sent to me, triggers Parallels to open which is annoying when I don't want it to and I have to wait until Windows XP has loaded before I can close it again. Is there any way to set up Parallels so that it won't open without first asking for confirmation? Thanks.