Have a look here:
unique hardware id c# - Google Search[
^] - there are loads of examples of getting an hardware ID there - you can use that eitehr as your "serial" of use a hashing function to generate a "serial number like" value.
But be aware: if I pay for software, and it later craps out because my HDD has been replaced, or my network card died; and an image of the previous reloaded I'm not going to be a happy bunny. I've had it happen before, and demanded my money back - and never, ever bought software from them again. Anti-protection mechanisms generally cost more than they save, and aren't very protective. Adobe spend a small fortune protecting Photoshop, only for cracked versions to appear on the torrent site the same day a new version is released...