OE6 is not recognizing mail on ISP server

[XP Home SP1, OE6 SP1]
I recently upgraded to ADSL and OE6 is now failing to recognise about 10% of e-mail on the server. I can see it with Mailwasher and read it, but when I switch to OE it says "No new messages ". All this is on my HP Pavilion desktop. The files are not large (all less than 20kb) and contain no attachments.)

When I look using my HP Pavilion notebook it sees and downloads the 'stuck' files.

Comparing settings for OE6 and IE6 between machines reveals no differences. Disabling firewall, ant-virus and pop-upstopper makes no difference.

Has anyone any experience of this problem? Are there any known apps or settings which may be responsible?

Thanks.

 

As I stated in the original post, I have tried disabling my anti-virus (Norton 2004) but this made no difference. The files are still stuck on the server.