plz. guide me to extract signature of a trojan for AVG

[the_gr8_rules]

Hello all,
just came to know about the way to make virus/trojan etc. undetectable.
so i'm trying...
i download a trojan and my AVG antivirus detected it.
the server file which this trojan makes is 7 kb.
so i started by splitting the file...
finally i got 1650 bytes of detectable portion.
(since doing it for the first time so not pretty clear about the things...)

then i open that 1650 byte file in Hex editor. There were around 83 lines. One by one i replaced all the lines with NULL and found that out of these 83...21 doesn't have the detectable signature.
What should i do next ???
still 62 lines there...i don't think checking for each byte would be a great idea at this time...
i must be missing something....please guide me


regards