> What have I done wrong?

Well, lets start by looking at the DATSHEET for the 12F675...

...there you will discover it hasn't got a PORTA, so...

c = PORTA

...isn't a valid statement.

Read the DATASHEET, find out what it's supposed to be, and try again... also don't forget (a) your TRIS statement, and (b) doublecheck to see if you need to turn off any any analogue functions on multiplexed pins...