This crackme can be found here.

I simply try to run the program which asks for a serial number. Writing a wrong one and pressing "Check" does nothing.

Loading the program into IDA Pro and searching for GetDlgItemTextA returns only one reference which is in a function named 'DialogFunc':

.text:00401028 ; BOOL __stdcall DialogFunc(HWND,UINT,WPARAM,LPARAM)
.text:00401028 DialogFunc proc near ; DATA XREF: start+Eo
.text:00401028
.text:00401028 hDlg = dword ptr 8
.text:00401028 arg_4 = dword ptr 0Ch
.text:00401028 arg_8 = dword ptr 10h
.text:00401028
.text:00401028 push ebp
.text:00401029 mov ebp, esp
.text:0040102B cmp [ebp+arg_4], 111h
.text:00401032 jnz short loc_40109C
.text:00401034 cmp [ebp+arg_8], 3EBh
.text:0040103B jnz short loc_401087
.text:0040103D lea eax, String1
.text:00401043 push 0Ah ; nMaxCount
.text:00401045 push eax ; lpString
.text:00401046 push 3E9h ; nIDDlgItem
.text:0040104B push [ebp+hDlg] ; hDlg
.text:0040104E call GetDlgItemTextA
.text:00401053 or eax, eax
.text:00401055 jz short loc_4010AC
.text:00401057 lea ebx, String1
.text:0040105D lea edx, String2 ; "Serial"
.text:00401063 push edx ; lpString2
.text:00401064 push ebx ; lpString1
.text:00401065 call lstrcmpA
.text:0040106A or eax, eax
.text:0040106C jnz short loc_401085
.text:0040106E lea edx, Text ; "Registered!!!"
.text:00401074 lea ecx, Caption ; "Project PolyPhemous - NTS - Crackme2"
.text:0040107A push 0 ; uType
.text:0040107C push ecx ; lpCaption
.text:0040107D push edx ; lpText
.text:0040107E push 0 ; hWnd
.text:00401080 call MessageBoxA
.text:00401085
.text:00401085 loc_401085: ; CODE XREF: DialogFunc+44j
.text:00401085 jmp short loc_4010AC
.text:00401087 ; ---------------------------------------------------------------------------
.text:00401087
.text:00401087 loc_401087: ; CODE XREF: DialogFunc+13j
.text:00401087 cmp [ebp+arg_8], 3ECh
.text:0040108E jnz short loc_4010AC
.text:00401090 push 0 ; nResult
.text:00401092 push [ebp+hDlg] ; hDlg
.text:00401095 call EndDialog
.text:0040109A jmp short loc_4010AC
Denne korte funktion henter først indholdet af tekst feltet og udfører derefter dette tjek:.text:00401057 lea ebx, String1
.text:0040105D lea edx, String2 ; "Serial"
.text:00401063 push edx ; lpString2
.text:00401064 push ebx ; lpString1
.text:00401065 call lstrcmpA


This short function retrieves the content of the text field and performs this check:

.text:00401057 lea ebx, String1
.text:0040105D lea edx, String2 ; "Serial"
.text:00401063 push edx ; lpString2
.text:00401064 push ebx ; lpString1
.text:00401065 call lstrcmpA



So it seems like the serial is 'Serial'.