How to reinstall the uninstalled system apps (that arent availble on Google Play)

What if you accidentally deleted some important apps and couldn't install them back because Google Play doesn't have them? For example android system, com.android dun server and com.android.lgsetup wizard (i.e. any prebuilt system apps). And every few seconds this message would appear: Sorry! The process com.android.phone has stopped unexpectedly. Please try again. And I cannot even go to the browser or Task Killer because every time I do, it would force quit. How can I reinstall those apps? I think I cleared all data for those but I did not clear the browser and so why can't I open the browser? Do I really need to send it for repair or can I fix it? Device: Nokia-X