"Open file with" dialog upon startup?

You likely have a file named Program in the root of one of your disks; for example, C:\Program.

You also have a startup entry with missing quotation marks, pointing to C:\Program Files\Random App where it should be "C:\Program Files\Random App". Such an entry lacking quotes can be parsed in two different ways – in this order:

1. Path "C:\Program", two arguments "Files\Random" and "App";
2. Path "C:\Program Files\Random App", zero arguments.

This is a mistake by the program developers, but it usually goes unnoticed due to file existence checks done by Windows – it just goes to the second option most of the time. However, when "C:\Program" does exist, the first option becomes valid as well, and Windows attempts to open the file.

Windows XP used to warn the user about the existence of such files at logon time. I haven't tested if Windows 7 displays one, though.

As for how the file was created in the first place – the cause might be similar; even fairly recent programs (and, well, users) sometimes have the 1970's idea that File Names Shall Not Have Spaces...

We can find where the file is located. I don't know if this will work in your case (for this mysterious Program file), but it should for any other real file.

1. Download and extract Process Explorer
2. Log out and log back in to have the Open with window appear (do not close it)
3. Run Process Explorer
4. Right click rundll32.exe in the list of processes

5. In the Command line: box, you should see the following:

   "C:\Windows\system32\rundll32.exe" C:\Windows\system32\shell32.dll,OpenAs_RunDLL <directory>\file
   

   Where <directory> is the location of the file (e.g. Program).

The directory may give you some context as to where this issue stems from. You should be able to find out where it is auto-starting using Autoruns and removing the proper entry. If not, simply deleting the file may do the trick.