Sometimes, for unknown reasons, VS 2008 creates solution files led by a newline.
Microsoft Visual Studio Solution File, Format Version 10.00 # Visual Studio 2008 [...]
This happened on various machines, and I have no idea why this is. A Google search did not yield any useful results.
Now, why do I worry about this? Because I can't open these solutions in Windows Explorer. I have to open VS, select File -> Open -> Solution and it works fine. But to open solutions from within Explorer, I have to edit the sln file and remove the leading newline.
Edit: After Leom's suggestion I tested a few times and found that the issue is solely dependent on the leading newline.
Edit: Interesting: VS 2010 displays the same behavior the other way around: It only works with a leading newline! At least on some solutions - on other solutions it's exactly the same as with VS 2008. I'm confused.
I too have a blank line in my solution files, but I do not have this issue.
I did some digging, it appears that solution files actually launch the Visual Studio Version Selector, and the first 2 lines (not sure if it the actual first 2 lines, or the first 2 lines of text) of the .sln file act as the input parameters for this selector.
There could have been a possible corruption of this executable.
In addition, you may try in Visual Studio Tools->Options->Environment->General and choose the "Restore File Assoications".