Have you checked that the file isn't actually read-only through Windows? In Win7 you can find it here: C:\Users\AppData\Roaming\Microsoft\Excel\XLSTART\Personal.xlsb (if you don't have a 'Roaming' folder, replace that with 'Local'). Right-click the file and check if the 'Read-only' tick-box is ticked. If it is, un-tick it.
If that's not the problem, are you trying to open a second instance of Excel? If so, the first instance will 'lock' Personal.xlsb so it is then read-only for the second.
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