Thomas, after renaming the main app folder do you delete or rename all the subfolders?If you have a V3 on your comptuer that's not an issue at all. Even with V4 it's not an issue as long as you rename the apps. C1 V4 and C1 V5 are simply named "Capture One". So an installation of V5 would overwrite/replace V4 (maybe this is the reason for the trouble mentioned above). But if you rename (V4-) "Capture One" to "Capture One 4" the V5 version will not overwrite V4. I am currently running V3.7.8, V4.8.3 and V5.0 on the same machine, the same drive/folder and the same OS. No trouble at all.