Run the crackme a few times to know about its protection. Here's one important info that I have found out by running it a few time, if you don't that means you are not alert enough:

We MUST enter INTEGER(not sure if really integer, what I am sure is that we must enter numbers that are less than 6 in length) into the text field

Disassemble the crackme with W32Dasm, go to Import Function window, search for MSVBVM60.rtcMsgBox(well, it does show a messagebox if we enter wrong serial into the text field, don't it? It is also a VB6 program and VB6 program display messagebox with this API) Double click on it until you reach the following code:

* Reference To: MSVBVB60.rtcMsgBox, Ord:0253h
                                                                |
:0040240D FF1524104000                     Call dword ptr [00401024]

Now, we are close to the protection. Scroll up until you see the following code which is very interesting to us:

:004023B8 6681FE870B                         cmp si, 0B87

Does that look interesting to you? Will the 0B87 be the correct serial(or code) for this crackme. Yeah, it is, but not in this format? You ask me what I mean by "format"? Ok, the 0B87 is in hexadecimal form, and the we have to convert it into decimal form before the crackme will accept it. Exit W32Dasm, load Windows Calculator(or even better, SoftIce or LaZaRuS's modified Windows Calculator), select View > Scientific, at the radio menus, select Hex, enter 0B87, change the radio menu from Hex into Dec and you will see the 0B87 number in decimal form. Enter that decimal number into the text field of the crackme, it will say thanks or something like that. Well, that's all for now, have phun!