Probably fonts. Open FontBook and deactivate any Helvetica fonts that aren't required by the system. Helvetica Fractions can cause problems. Try setting Safari and Mail to a font other than Helvetica; perhaps Verdana.
Do you have any other user fonts placed in System/Library/Fonts? Move them out and make your own fonts folder in the first layer of your hard drive directory. Link to them from there.
Hope that helps.