Thanks for all the responses.

Elprup, I'll consider that as a back-up plan.

Shg, do you know if users will receive the exact same message for a CACert? I'd rather go with CACert since they're free, but I'm wondering what the differences might be. For instance, I'm guessing most firewalls allow verisign now, but might not allow cacert.

Roy, I'm creating it in Excel 2007 and saving in 97-2003 compatibility mode. The users could be using any version of Excel. By your statement, do you mean that a Digital Signature for the VBA would still remain valid if a user changed spreadsheet values and saved? Do you know any circumstances that could cause a digital signature to become invalid if you have the vba locked (barring a user guessing the password)?