No, no, no, M4 and V4 do not need to be in the main Poser Library.  Only morph packs and the base files should be together.  This is a fallacy that's been circulated for way too long.  The scenario for Win 7 installation, select the option which will place a Poser runtime into a shared document folder.  All content will now reside away from Programs File folder.  Since the original WinXP content file already occupies the Download folder, simply transfer that runtime to the new Download folder under the Win7 environment. 

Now, you are aware that unless you are building this new computer from scratch, any new computer you purchase will have Win8 installed.