Go to options > attachments > incoming and delete everything related to MS office documents. Now restart Thunderbird to make sure your deletions are actually recorded in the program.

Next time you open an office document you will be asked what to do with it and you can specify the application you desire to open it. Note the associated applications you get to use is offered up by the operating system, not Thunderbird. So if the office applications are not offered you might need to do a repair install of office to fix associations in Windows.

Thanks so much Matt, have done this and deleted everything related to MS Office, but still can't open attachments. Will look into repair install of Office if I can find that. Thank you, appreciated.