Font changed by Conditional Formatting
 

I have created conditional formatting in Excel 2007 to change the colour of
text under certain conditions, but the colour AND font change. The font of
the cell in "normal" conditions is Arial, changed to Cambria for reasons
unknown. This is a problem because Cambria 10 pt appears to be smaller than
Arial 10pt. I've never knowingly used Cambria, and it is not possible to
control the font within the "format cells" dialogue box anyway, so where has
this font been picked up from. More importantly, how do I get rid of it?