it sounds as if the 500 watt psu you purchased is an incorrect type for your computer. Same for the work one, as it does not fit in your pc.
If you advise the make and model of your computer (if unknown, advise the mobo's model number), someone here will advise you which type of psu you require.
Also advise the make, model number, and wattage of the failed psu.
When purchasing a psu, the type should be the same as the one it is replacing, and the wattage the same or (preferably) greater than before. If the wattage is less, it may not be enough.
Regarding the old psu, if nothing starts, it is worth opening it up and blowing out any muck with a hairdryer set to cold and maximum blast. Also clean its fan. Generally there is a small internal fuse, that you should also check is ok.
Good Luck - Keep us posted.
message edited by Mike Newcomb