You can write your version of cmpstr function in assembly. For example:
abc db "abc",0
def db "def",0
...
mov ecx,3 #the length of the abc and def strings
cld #set the direction flag so that EDI and ESI will increase using repe
mov esi, offset [abc] #moves address of abc string into esi
mov edi, offset [def] #exact syntax may differ depending on assembler you use
#I am not exactly sure what MASM accepts but certainly something similar to this
repe cmpsb #repeat compare [esi] with [edi] until ecx!=0 and current chars in strings match
#edi and esi increase for each repetition, so pointing to the next char
cmp ecx,0 #test if the above command passed until the end of strings
je strings_are_equal #if yes then strings are equal
# here print the message that strings are not equal (i.e. invoke MessageBox)
jmp end
strings_not_equal:
# here print the message that strings are equal (i.e. invoke MessageBox)
end: