The IF statement per se should not and I'm pretty sure does not twiddle the pin but if what you you're doing is somehing like...
Code:
If PortC.4 = 0 THEN
   PortC.1 = 1
ENDIF
...and there happens to be analog functions on the pins which you haven't disabled then the actual state of PortC.4 isn't read correctly and you're getting into the classic RMW issue where the "invalid" state of PortC.4 is being written back to PortC.4 when the write operation to PortC.1 occurs but without seeing the rest of the code it's hard to say for sure.

/Henrik.