Is there an easy way to multiply fractions in assembly language using a PIC18F4550 instruction set?

I need to convert from Fahrenheit to Celsius and from Celsius to Fahrenheit.

 

You don't need fractions, you should be using three simple integer ops in sequence.

C to F: /5, *9, +32

F to C: -32, /9, *5

If you need fractional degree resolution, then work in tenths or hundredths of a degree, still as an integer. (And scale the 32 offset to 320 or 3200).