Just hooked up a clean system with XP Pro SP2, only Opera installed. Mail worked fine.
Installed Avast 4.1, no problems.
Upgraded to 4.5, same problem appeared.
Used the same system. But now only with OE and Avast, no problem.
Sure looks like it is a problem with Opera and Avast to me.
btw, I'm using Avast Pro, not that I think it makes a difference with the home version.