Start *Windows'* safe mode with networking enabled - XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still In Windows safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Does problem go away? Please reply to let us know results.

Hi, and thanks for getting back so quickly. Yes by running in both safe modes as suggested does appear to cure the fault. Regards

In that case, (still in Windows safe mode) ... start Thunderbird normally

   -- If problem is still gone, then cause is a program loaded during windows startup.  Possibilities include: antivirus SW, virus/malware, background downloads such as program updates
   -- If problem is NOT gone, then cause is likely a Thunderbird addon - eliminate them by disabling each one at a time in Tools | addons | extensions and restarting

- If results are unclear ... possibilities include temporary conditions such as contention from other running programs, downloads related to windows update, ...

Also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues and https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

Let is know results please

Went through in the order you suggested. It is really tedious but I eventually found the culprit.

McAfee virus and spyware scanning.

In the settings the function "Use Active Protection" was checked. Unchecked this and all is well. Thank you very very much.

Thanks for the results feedback