Icky problem! If this were me, I'd have kitty checked out by the
vet since this is a new behavior, so it is probably health-related.  
Then I'd add another litter box or two depending on the size of
your house, and for the odor, I'd call a professional heating duct
cleaner company and tell them what happened and they'd have the
appropriate cleaning/deodorizing treatment for it.  For other pee
"targets" clean thoroughly with enzymic liquid cleaners (sold in
pet stores like Petco) by thoroughly saturating the areas, letting
it dry, then repeat if necessary.

You need to go to a pet supply store (Petco, PetSmart, et al.) and buy
an enzymatic cleaner. Be aware that these cleansers vary widely in
effectiveness. A proper enzymatic cleaner will actually eat the uric
acid crystals. There are also special black lights sold in pet stores
that, in a darkened room, will detect urine deposits that you ordinarily
wouldn't be able to see or smell; this helps ensure that everything is
cleaned up. Clean down into the ducts, and clean the gratings as well.
Note that it might take more than one cleaning to get rid of the smell
entirely. Also make sure your cat is confined to a different area of the
house while you're doing the cleaning. If the urine has gone so far  
into the ducts that it's impossible to reach, contact a heating and
cooling professional and ask for his help; if it's down that far, he'll
have to come over and take care of cleaning the ducts from that point on.

Once that's finished, put the grates back on, and spray them and the
area around them with a cat repellent sp that your cat won't want to
come back there. Otherwise he might decide to make a repeat performance
in your air registers.

HTH.