Well a 877A will do the trick for sure, but if you had loops or only a couple doors and windows it is overkill.
Interrupt are OK, put the PIC to sleep and let it wake up on the interrupt. But you must also check for keypad data, serial data and other things. Or is the PC the whole alarm?
But to be honest, I would poll it every 10, 20 60 seconds, check for serial data in that loop realtime and just run the PIC. The power saving is not worth it, code will be easier, and you have other thing to service like the serial data.
So how many wires do you have? Did you ohm them out and label the wires? Get that part done first. Come back with that for ideas on the chip and methods.
Also, I have to look, not sure if I used a MAX232 on mine. It is old and I forgot to bring it home. I either inverted the bits in software or used 2 transistors to fake the MAX232 (voltage level shifting). If you have a max232, then use it. But I doubt I had one as back then you need a bunch of CAPs for use it.
EDIT: Opps. Just caught the nine wires and 7 zones. OK, you do not need a 40 pin PIC. But your call. The 16F88 and an ICD2/Inchworm unit and you are set. And use the internal oscillator, less parts. Are you doing it in assembler?