What is wrong?
 

I have an exe packed with a UPX packer and PEiD idents that packer. It runs ok. I want to modify something and let's start to unpack it.

Unpacking is done, but when I test to run it, programs says not found a .dll file, I used UPX to compress it again, but software says not found .dll like when it was packed.

What was wrong there?

Maybe it's a CRC or something like that, or a check of the filesize which of course both differs after unpacking...

MiniMind

Check your File for [Overlay] maybe during the Unpacking you lost the Overlay, if PEiD Show you that "UPX x.x [Overlay]" it seems that you had overlay on the file.

Best way to unpack such thing is using Resource Tuner Embedded UPX Unpacker, Just open it in Resource Tuner and then Save your File.