The active version of the Helvetica font is not the default Apple system font. You've removed Helvetica.dfont from /Library/Fonts, and/or you have another version of Helvetica open (like in Suitcase or another font management program). The widgets are designed to use Helvetica.dfont, and you need to keep that one active. Some other versions of Helvetica may conflict with it, but it's usually manageable.