Cannot find text in files

Maybe because of my poor english, you misunderstood me. I didn't expect to crack the program by deleting the text file. I just ment that the text strings (like "the serial number is incorrect") i only found in that text file, and not in any exeutable.
I know that i have to find the code that checks the serial number, but i cannot, because one DLL is 99% encrypted. (Dialog ref. in Win32dsam shows that there are some bytes about serial numbers, but no offsets.
Greetings