OS 10.5 - Problem with Mail crashing

I have a problem with Mail crashing every time I open it.

I'm running Mac OS 10.5.5 with basic programs (very few 3rd party apps), the problem with Mail began a week or so ago either after installing the 10.5.5 update, or the security patch that followed a little while after.

Very annoying, I can look at my mail, send mail &c in my ISPs webmail, but I can't refer to my saved e-mails.

What's the problem? How do I solve it, and why did Apple allow this to happen?

(This isn't the same error as the Mobile Me error of a few weeks ago, I'm not a Mobile Me subscriber, this is Mail connecting to a POP account at an ISP, although it crashes immediately, before it's had time to begin trying to download mail).