I need a program to edit exe files. For example to rewrite .text section in assembly or edit data .section. Do you know anything free suitable for me ?
While IDA Pro is by fast the best, as Michael recommends, it's not free. Try something like Ollydbg instead.
Never heard of PECOFF files, but I'm guessing since they're exe files you'll need a hex editor. I use bvi for hex editing, but it helps to know vi editing commands to use it.
You need to search for a "windows disassembler".
IDA Pro is one of them.