Hello Dreamboat - the mind boggles!
I have solved the problem.
I first tried run "sfc /scannow" on the suggestion of a.n.other but it didn't find the file on my system or re-install it from my Windows CD. It's clearly not part of Windows XP Home Edition Service Pack 1.
However, having downloaded it from
www.dll-files.com, I didn't realise it has to be registered before it becomes active. This is done by clicking Start\run and then typing C:\WINDOWS\system32\regsvr32.exe Msstdfmt.dll
Thanks for your interest.