I had the same problem with opening tiff files in Photoshop and I tracked down a rather obscure fix. It worked for me, so I will explain it here in the hopes it works for you. Be warned it's a bit of a crazy dance:
1. Go into My Computer>Tools>Folder Options>File Types;
2. Select your first file type (.pln) and click the "Change" button. Select a program - any program, from the list. Do this a few times until the "Restore" button in the Folder Options dialog finally changes into "Advanced." This can take up to 10 attempts! (Crazy!
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3. Once the Advanced button becomes available on the File Types tab, click it. Select "open" from the Actions box. and click the "Edit.." button;
4. Click the "Browse.." button and browse to your archicad.exe file. Double-click it and the "Application used to perform action" box should update with the path to archicad.exe. You may want to add " "%1" " to the end of that path, which basically means open file;
5. You can also change the icon, using the Change Icon... button. In the next dialog hit Browse... and browse to your archicad.exe file where you will be presented with a variety of icons. Select the appropriate icon and hit OK.
6. Click OK to go back to the "Edit File Type" window. Click OK again to return to the File Types tab. ArchiCAD 10 should now be listed as the program we use to open this file type, not in the list, but at the bottom of the dialog. Click 'Close';
If it doesn't work first time, then...
7. Right click on the file > Open With > Choose Program... > (choose AC10) and check 'Always use the selected program to open this kind of file'.
Hope that helps.
Cheers,
Link.