I'd first ensure you have a good and complete backup, lest something here go wrong. (It shouldn't, but there's already something slightly weird occurring here, so it's best to be cautious with your data.)
First test: copy the the credentials and settings to a text file or a yellow sticky, delete the two problematic mail accounts, exit Mail.app, re-launch Mail.app, and re-create the two accounts. See if this then avoids the crashes.
If that test fails and the crashes persist, try downloading and applying the OS X 10.6.8 combo kit from Apple over your existing installation — this won't cause data loss — and see if that clears things.
Hi. Thanks for your reply. I've only just had some time to look at this. I've reinstalled 10.6.3 from my CD and got the same problem. I've upgraded to 10.6.8. Same problem. I've created a 'test' user and logged into that and created my two email accounts. Same problem. I've closed all other open applications and same problem.
Mail works fine, but when I come to un-enable an account Mail either crashes or the Mail Preferences window won't shut down (the account is un-enabled but I can't close the stupid Preferences window).
I've got an Apple Tech support person phoning me at 12. I'll let you know how I get on.