In a multiuser environment, Win7 64 bit Windows Explorer Preview pane is locking files and deleting the existing file when an overwrite is attempted. The newer version isn't allowed to be written to the folder. Is there any way for a user to be previewing a file, while another user is overwriting the same file. How can Windows Explorer be prevented from deleting files?

An "overwrite" is really just a delete and re-write. To answer the question "Is there any way for a user to be previewing a file, while another user is overwriting the same file?", the short answer is no. It can be done with database solutions, but not in the current implementation of NTFS (as far as I know).