Hi Paul,
The default font in the CaseComplete interface is Segoe UI and cannot be changed. However, any report you generate should honor the font that is specified in the report template anywhere that Segoe UI is used.
There is, however, one sticky exception if the project was created on an XP machine that did not have the Segoe UI font installed and then later updated on machines that do have Segoe UI installed.
Do you think this might be the situation you're in? If so, I might be able to give you a workaround.