Going on from whaat Richard has very rightly stated, you will certainly be able to use the transformer to step up voltage, But most microwaves are about 700 - 1000W. So a 220V winding will be passing about 3 or 4A. The winding resistance of the 220V winding will not be much at that voltage but will become appreciable if you try to pass that much current through it at 25V. the same applies for the 2000V winding if used at 220V. As long as you don't want to get a lot of power out of it you should be OK, I'd say you'd be able to get 100W maybe at the most. Can you measure the windings resistances and advise?Tip - if you want to do this at appreciable powers, look for an old UPS transformer, preferably from one that had 2 x 12V batteries inside it. You should find that it will have a transformer inside that will do just what you want. If you're in Australia I have several lying around that you'd be welcome to.If you're not looking for as much power as this just buy a suitable mains step down trandformer - it will work fine the other way round, something like this.Cheers // David