Vista - New Messages Not Displayed with Windows XP

I run Windows Live Mail on three different computers, each with nearly
identical configurations, all using Windows XP with all of the latest
updates. On one, I will get an audio notification that I have new messages,
but the Inbox often does not display the new messages. I have to click on
another folder, like Sent Items, and then go back to the Inbox to be able to
see the newly received messages in the Inbox.

I have tried completely uninstalling all Windows Live components and then
reinstalling serveral times, plus completely deleting the message store so
that only messages kept on the Hotmail server are downloaded to my machine,
and nothing helps.

So, the only thing I can think is that perhaps there is a conflict between
Windows Live Mail and some program that I have on this machine only. Has
anybody run into a similar problem?

What's especially weird is that it doesn't do this 100% of the time.
Sometimes, after deleting a bunch of messages, for example, it will operate
exactly like it is supposed to for a while.

Frustrating. I'd appreciate any insight anyone has.