Excel 2007 - Conditional Formatting: Font reverts to Cambria

Question

Whenever I use conditional formatting the font changes to Cambria (size 10 or something). I can't change the font nor its size using the [home, conditional formatting, manage rules, edit rule, format, 'font'] tab. It's all greyed out! It looks like a bug? I've seen posts on this on the web but no answers. What now?

(Ps I'm using Windows XP SP2. We're going to upgrade to Windows 7 in a few months, which might make the problem go away.)