Newsgroup Messages

My newsgroup messages have disappeared from view from within Windows Mail. I'm running Vista Ultimate and everything has been fine until about 2 days ago. My newsgroups show that I have 'x' number of messages but none display at all. I've reset the odd newsgroup and requested the headers and it shows that it's downloading the headers but none show. Then when I've downloaded several thousand the only messages that then do display are ones that I've created or responded to.

Help, I'm missing nearly 250,000 messages from my newsgroups which act as a huge resource of information to me :(

Thanks in advance for any advice.