"Should I be upgrading or install into a new directory?"
Install into a new directory
Here's what the knowledge base says about error 115:
Question: Why do I get Error -115 during installation? Answer: Error -115 during installation usually means that the directory specified by the TEMP environment variable may be full of files. The installation routine stores temporary files in a directory specified by the TEMP environment variable. To determine whether your computer has a directory designated for temporary files, type set at the command prompt, and then check the values that MS-DOS displays for the TEMP variable. For example: TEMP=C:TEMP Check for the directory specified by the TEMP environment variable. If it does not exist, create one. If it exists, check the contents of that directory. Delete any temporary files with a .TMP extension contained in that directory. To avoid deleting a temporary file that is currently in use, quit all programs, including Windows, before deleting the files. If the TEMP directory is C:DOS, edit the AUTOEXEC.BAT file and change the TEMP variable to point to another directory such as: C:TEMP or C:WINDOWSTEMP If the directory you specified does not exist, create one. Consult your Windows manual for information on deleting files, creating directories and editing AUTOEXEC.BAT files.