I am trying to reverse a program that I own. I have run the .exe through Olldbg but am not sure where to look in the ASM code. I have read some tut's on where to look (break points), for example if the software asks for a license number and you put the wrong one in and a pop up occurs, then you can narrow it down. The only thing is with the software I own, it shows a registration tab, and another form appears, with a key number and an ID number that must be emailed to the programmer/company. Which I did when I bought the software. Anyway, I was messing around with it and put in a bad number, and no warning window appears, the reg form simply closes. At this point I have no idea where to look. I NOP'd out some code where I thought the registration process was, and recompiled but it crashed. Thoughts? Thanks!!!

GetDlgItemText ?

Also, GetWindowText and SendMessage (with WM_GETTEXT).

To sum up, you need to know where to look before changing ASM instructions. API hunting is a good start. If you are not familiar with the soft, or the technique (you need some training about which API is intersting for you etc..), may I suggest OllyDbg tutorials ?
cheers.

IDA it. Figure out the whole registration routine.

Randomly changing code around in a debugger usually doesn't accomplish much.