@swampwiz:My guess is that the tool you are using to set up associations is not quoting the file name, which it must do. Use the builtin win7 tool to set the association. I just did it and it works find, even if the file name has spaces in it.
Right click on an epub (or whatever) and select properties. Near the top there will be a section "Open with ...". Click the button, press "Browse" and navigate to calibre-viewer if it isn't offered as an option then press OK, select calibre-viewer. See the attached image.
As I said, I did this and it works fine. You can see that the file name of the book displayed in the attached image has spaces in it, and it opens.
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