I assume that your email client is Outlook Express. It's possible that your computer's registry has lost the entry defining it as the default email client, so your scanner program doesn't know which program to use. Open Outlook Express and click on 'Create Mail'. If you see a message stating that Outlook Express is not your default email client, click the button which will make it so.
If that doesn't work, uninstall and reinstall the software which came with your scanner.
Chris