Ivanovich said:
In case someone else has this issue, this reg file solves it, need to put proper path to executable though.

Code:
Windows Registry Editor Version 5.00 

[HKEY_CURRENT_USER\SOFTWARE\Classes\Applications\xnviewmp.exe\shell\open\command]
@="\"C:\\Portable\\XnViewMP\\xnviewmp.exe\" \"%1\""
Perhaps stop fiddling with the registry editor and try a Repair Install and go from there? As said, Windows natively supports bitmaps (.bmp) so the issue is either the program used to open the files, or somewhere down the line Windows hosed itself for bmp files.

With that, out of curiosity, are jpegs experiencing the same issue? What about .tiff files?