The Default measurement units in excel have been reset to inches since I had to re-install win 7 and thus MS Office Home & Student 2013.
This has made all my printing margins incorrect, as the number has ben retained e.g. 0.75 cm is now held as 0.75 ".

Perhaps change your regional settings ( can't check, I'm working on a Linux machine)

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I don't know if it is the same in your version as in Excel 2007 but if you have an Office Button (round button at top left), click it and you'll get the "Excel Options" window.
Select "Advanced", "Display" and there you'll have "Ruler Units".
This can be changed to a.o. centimeters.
Now I don't know if that'll change the margins but you can certainly try that.