Ive been trying to use cheat codes on this game but they are not working. Ive tried codes such as DEADDACK for all levels unlocked but it keeps saying the code is rejected. Anyone else got this problem?

Probably because DEADDACK isn't a valid code for the PC version. Try these instead and shout at whoever gave you bad codes.

Here are codes that are working on the GOG version so far as I have tried them:

Passwords
• This is a list of all the cheat codes compatible with Star Wars: Rogue Squadron
To enter the code, click on SETTINGS, then click on the box below where it says PASSCODES and type the code.
Password Effect
LEIAWRKOUT Activates Force Feedback for PC Joysticks.
HIKEN Bonus secondary weapons
NEUC Destroy all Imperial Ships on radar
CNDDOBY Displays a picture of the development team.
USEDAFORCE or FSSRNB or JKQCNMH Enable all ships for levels. Might also lock-on target with Secondary Weapon (NOT TESTED)
WOOKIEPELT or PKDWXE or CRBSIFA Enables Millennium Falcon
GOODGUYTIE or AAAAFNPDJY Enables the TIE Interceptor. Must have Millennium Falcon enabled first (Goto Millennium Falcon in hangar, then hold down or back)
IAMDOLLY Gives you infinite lives.
DIRECTOR or EAAAABUMFU Let's you view all the cutscenes. PC: Type this in, go to High Scores, and choose "At The Movies."
MAESTRO Lets you listen to all sound themes (Goto High Scores, then choose Concert Hall)
GUNDARK Modifies Force Feedback control for PC Joysticks.
BERGLOWE or WUTZI Modify Luke's face
CHICKEN or OTOOJKH Play as an AT-ST in any level except Taloraan.
LOKJOT Repair damage
TOUGHGUY or NAKASGMQ or MLVBHF Technology Upgrade
WOOKIYRWMW or AAAABDBOHP or NDAAFXWSFQ or EAAAABYCBA Unknown
NUMBERTWO Unlimited secondary weapons
WIMPAMI Unlock all bonus missions
OOMPAWAMPA or GCTHNME Unlocks all bonus missions
ACE Unlocks Expert Mode
CREDITS or ONCHHZU View Credits.