At this point, everyone should remove the trust for the Diginotar Root CA. I guess most people know how to do this around here, but just for informative purposes:
First, visit their web site to ensure their root certificate is in your certificate store:
On Mac OS X go to Applications, Utilities, open Keychain Access. Click on System Roots, then find the "Diginotar Root CA". Select it then do CMD-I. Open the Trust Panel and choose "When using this Certificate Never Trust" instead of System Defaults. Close the window, enter an admin password and close Keychain Access.
On Windows it's a bit more complex (no, really?). Start, Run, mmc.exe, OK. Confirm UAC if under Windows 7 with admin password if required. If you're under Windows XP, relog to an administrator account first. Then go to File, Add/Remove Snap-in, find the Certificates snap-in, click on it, then add. Select the Computer Account and local computer. Then open Trusted Root Certification Authorities, Certificates, find the "DigiNotar Root CA", right-click on it, properties and choose "Disable all purposes for this certificate".
Make sure you don't delete the certificate, as it would just get re-approved.