If you install both versions of Lightwave, you have do so in a particular way, otherwise a few files get overwritten and some things in Lightwave 7.5 won't work. I'm still pretty new to Lightwave so I can't remember what they were. But it happened to me. I had 6.5 on my machine and then I installed 7.5. I then ran 6.5 because I knew it best. But as soon as I did, it changed something in 7.5. They had some instructions on NewTek about how to set them both up, if you wanted to have them both. Not long after, I got a new pc and I only installed 7.5. Search the NewTek forum and you'll find information out about this.