What if you did it like this?
Code:
 ShadowC VAR byte
 ShadowC = PORTC

 ShadowC.0 = 1
 PORTC = ShadowC

 ShadowC.1 = 1
 PORTC = ShadowC

 ShadowC.2 = 1
 PORTC = ShadowC

 ShadowC.3 = 1
 PORTC = ShadowC