Your program might be interrupted only by an Hardware Interrupt, say from the USART module.

So, if you want a fast interrupt system, have a look at Darrels Instant Interrupt.

Then every time you have an RS-232 activity, the ISR will take care of the event. Store the incoming data in an array and do a compare byte-to-byte with a stored reference array, say in the EEPROM.

I am working on a similar project and may post a snipet as soon as I get it working.

Ioannis