First of all, you have to download OllyDbg, my favorite Windows debugger and copy EasyCrack.exe (in directory \Help\EasySerial\) to a working directory.
At first, start EasyCrack. A nice dialog box will appear. Enter your name and a serial (for example 123456) and click OK button. Harrr... "Entered serial is incorrect!"



Now, start OllyDbg and open EasyCrack.exe. It will take a moment. You will see a screen like this:



I won't explain you what are Stack, Registers, opcode and others. You should look at some assembler tutorial and you will learn it. You don't need to know these terms to follow this tutorial. Move your mouse to Assembly listing and right click on it. A popup menu will appear. Choose "Search for" and "Binary String". In new dialog type in ASCII edit box: "Entered serial" (a part of "Entered serial is incorrect!" message) and click OK. Assembly listing window will move you to the message. Now you have to right-click on the line with 'ASCII: "Entered serial i"', select "Find references to" and "Selected address". A new window will appear with references to the message. The first reference is what we need, so double-click on it. Assembly listing window will bring you right to the place in code, where MessageBox is created.
Take look at this commented piece of code:



We will set a breakpoint. Double-click on CALL EASYCRAC.004014D4 (Compare strings) opcode (this is the "E8 0FF9FFFF" part of line). The address 00401BC0 should be red now (). From OllyDbg main menu select "Debug" and "Run" (or press F9). Again, fill in your name (I will type "migeel") and some numbers as serial (I will type "123456") and click OK. OllyDbg will appear with application stopped at our breakpoint. It is stopped right on call to StringCompare function. Look at Registers window: in register EAX is the first string and in EDX is the second string. As you see, EDX is serial, we typed, so EAX must be the correct serial. If you typed "migeel" as name, you'll have 243 as the correct serial. Now press Ctrl-F2 (Restart) and F9 again. Now type entered name (I typed "migeel") and serial, which was shown in ASCII EAX (I had "243").
Thank you for registering Easy Serial ;-)