If you know the Hex for the instructions you are looking for - or know the offset - it should mean you can just type your NOP hex codes over the ones there. HHD Hex Editor that certainly applies - I had some jpeg images once that had the header corrupted by one hex digit that had to be corrected (dodgy 3rd party jpeg converter). Also had to alter some bluetooth drivers to work with a specific model of dongle - followed some instructions on a forum and was there.

If you ever need to get at parts of a program to extend it or get at unexposed functionality, investigate dll injection. There are several good articles on that in codeproject.

I'm a little concerned about the example you are using in your post though. Things about hacking serial numbers may directly effect people's livelyhood whom post and read here - so if a mod did delete the post it was probably because they thought it was a bit insensitive.

Fishing pole:
#asm has some excellent links.

The Newbiez Project used to have lots of good tutorials, but sadly they've closed up shop. You can take up the search (go "Zur Seite", then tutorials, then choose a topic in the top bar).

Fish:
if you've found the offending instruction at address <a>, file offset can be calculated as follows: section_offset = a - code_section_virtual_RVA - module_load_address (0x400000 for EXE); offset = code_section_raw_data_offset + section_offset.

Addendum:
regrettably, a lot of interesting and legitimate programming knowledge is tarred by the "hacker" brush. As to someone's livelyhood depending on people not knowing how to crack a simple serial protection scheme: given an opportunity, even non-kleptomaniacs can be driven to "steal". When parking a bike unattended in a rough part of town without a lock, it can be written off. Just as you want to invest in a decent bike lock to secure it, coders should know how to protect themselves. If you do not, you are better served learning how, rather than trying to prevent the inevitable spread of knowledge.

*sigh* It's a shame that morals come up every time reverse engineering is mentioned. It's not like we're providing a step-by-step instruction booklet how to steal (or build a nuke), which would be bad - this information has valid use.

/* fixed links */

[Edited by - Jan Wassenberg on January 18, 2005 9:14:59 AM]

I can understand the need to learn what you're wanting to do, purely for the purposes of ensuring that your own code is not easily hacked by others. (I work in the security industry)
I myself have often checked that say, database connection passwords are not visible in plain text in the compiled executable, but are instead put together from bits and pieces.
I'd still prefer not to help though sorry, not that I have a lot to offer here anyway.