My crystal ball tells me you're using NasmIDE.
This error message is *not* from Nasm. Unless I'm mistaken, it's a "Pascal thing". I was under the impression that it was fixed in recent versions(?). In any case, it's from NasmIDE not finding Nasm - either because it hasn't been filled in in NasmIDE's "options", or for some other reason. Your "Portuguese" source suggests a possible reason - long filenames in the path. You should actually be able to use 11 characters - 8.3 (paths can have a "dot" in 'em, too!). Filenames/pathnames longer than 8.3 are "decadent", in my opinion, and filenames/pathnames with spaces in 'em are pure EVIL!!! Winduhs kindly provides you with "Program Files", by default... Spaces in filenames/pathnames will definitely mess Nasm up - "%include" search paths, etc. If you're stuck with spaces, use the ugly, mangled, "short" name - "progra~1" (which is the "true name", in FAT fs, at least), and the problem should be solved.
Another possible problem is that NasmIDE won't run a Windows build of Nasm (usually called nasmw.exe). You'll need to use either "nasm16" (ugh!) or the djgpp, "32-bit dos" build - neither of which will handle long filenames, so you'll want to avoid 'em, or if you're already stuck with 'em, use the short name.
If by any chance you're using the CD with Jeff Duntemann's book, there are some known issues with the software - it copies off to disk with the "read-only" attribute set. Take a look at his page -
http://www.duntemann.com - for more info on that. I don't know if it applies to what's on the CD, but the examples downloaded from his site contain some errors (IMO) - mostly just cosmetic. Let me know if you want more info on that...
If I've completely mis-diagnosed the problem - it's *not* from NasmIDE - then I don't know what "spawno error code 3" is. Just like it says, it's "unknown" :)
Best,
Frank