Using Mozilla Thunderbird (current version is 38.5.1 and my OS is XP Pro).
Often, upon clicking “Send,” I get an error message: “Problem including file file:///C:/docume~1/(my name)/locals~1/temp/nsemail.html. Would you like to continue sending the message without this file?” If I click on “OK,” sometimes it goes through OK and sometimes it goes through with the message text deleted.
If you have XP Pro SP3 then it is a 32bit version - I believe the only XP Pro version that had a 64bit version was SP2 - the 64bit version only went to SP2, no SP3 release. By default then, if you have XP Pro SP3 it is 32 bit.