just get a new psu and the computer will be up and running again fine.
your existing psu is probably a load of crap anyways, psu with voltage selector switch on the back doesnt even have active power factor correction.
if you know what your doing (which i seriously doubt you do) then you can open up the psu and replace its internal fuse. since you set the input voltage to 110v the internal fuse would have blown since your feeding it 240v.
also every psu i encountered which has a selector switch for the voltage the switch is recessed and very small and needs to be set by sliding it, and usually fingers are too big to get to it (especially by accident) which is why i suspect this thread may be just a wind up.