Not my area of expertice, but I suspect it is not the file path of the attachment, but something to do with the viewer, ie 32 bit mode trying to execute 64 bit viewer.
If you have a windows 7 that works with one project, try downloading the attachment and uploading it to a problem project and see if you can open that attachment. If so, is it a different type, ie .txt vs .xls or .doc (or .xlsx or .docx). Sometimes its not the specific program but the add ins they try to load to do conversions that fail and do so in the background.
Next thing to try/check is turn UAC down to low or off. Problems can happen when invoked in the background and the error is mis-reported, ie should have been an access error not a path error.
local admin rights might also allow something to be created like a temp directory on an otherwise locked down C: drive. Path would fail for non-local admins since a path would not be created.
Good Luck