Email histories fixed.
It turned out that the culprit was Outlook.
To transfer out of windows mail, you need to transfer to Outlook and then onward to your selected email program. I had a triple boot setup, XP, Vista and Win 7. and was trying to transfer my email histories from Vista into Win 7.
I was running MS Office Home and Student, which does not have Outlook, so I loaded Outlook from my older Office XP Pro disk. I only installed Outlook, not the whole Office files and that was what caused the problem. My email histories transferred OK but were corrupted and I could not open them in Windows mail or Thunderbird under Win 7. In fact, they could not even be viewed.
I went back and loaded the whole of Office XP Pro and Outlook then did its transfer job correctly. I hope this helps someone caught in the same situation.
After dabbling in Thunderbird, I quite liked it and have decided to stay with it.
I perhaps should not be that enthusiastic about transferring everything to Win 7 RC, as shortly, the real Win 7 will be available and I understand an upgrade from the RC version is not possible.
Bobthequill