MYM is a very old program. It's bad business practice to rely on very old unsupported programs. Why not ask your accountant what he advices?
Of course, there's nothing wrong with Windows XP, even after April 8, 2014, if you don't connect it to Internet. So, on a not too old second-hand computer with XP, you could use the version you have now for years to come, if you prefer it. That's probably true for the timesheet software also.
Kees

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