ok, well before we can start , u need a hex-editor , a hex-editor lets u change the "contents" of an exe...
I will be using Hex-Workshop for this....

Download (clean app)

http://BreakPoint.He

k, downloaded (and installed of course) hex-workshop, then continue reading.....

ok, now, find ur trojan and right click on it, press the label that says: edit with hex workshop...

now a window (hex-workshop) will appear and u will see lots of strange code, some saying 4D5A and so on,,,
now we are gnna check which offsett the av detects (an offsett is ONE column of numbers , example: 4D5A is a offsett)

well, drag the slider to around the middle, mark the line of offsetts and drag till the bottom... when you've marked them, right click and press fill, here you write 00 and click ok,
now go to file , press save as and save as server UD 1.exe
now you scan it with ur exe, if it was detected, then we know that the offsett that makes it detected isnt in the half last part, so now we try to go from half, and like 10 linees up, and see if its detected,, if its detected,, continue experimenting and eventually u will find the right offsett....

ok, so lets say we found the offsett and the offsett was 0F5J , you should try to change it to like: 0F5I or 0F5K