With picasa and a host of other picture viewing programs, the default image viewer Microsoft's Picture and Fax Viewer kind of went into oblivion for me. One fine day, I wanted to use it for viewing images (out of pure nostalgia) and found out that it was not getting listed at all in my "Open with" menu. I also was not able to find it through any program. After googling, I found out that it is actually a DLL, shimgvw.dll and needs to be launched with rundll32.exe. So, I tried that. I typed
rundll32 shimgvw.dll,ImageView_Fullscreen
both in "Run" prompt as well as from the command line. Both gave me an error. "Missing entry ImageView_Fullscreen". None of the forums seem to have given a solution to fix this issue. After some more unlucky searches, I ended up with this command to be run. In the command line, run "regsvr32 /i shimgvw.dll" (without the quotes). It should say "registered successfully". Voila, my problem was solved. The viewer started appearing in my right click menu. I still do not know how it vanished in the first place. But, atleast I know how to bring it back.
