DISASSEMBLER AND DEBUGGER WANTED !

I NEED AN URGENTLY HELP !!!

WHO CAN TELL ME WHERE CAN I FIND A POWERFUL
SOFTWARE TOOL (DISASSEMBLER AND DEBUGGER) TO
MODIFY AN EXECUTABLE PROGRAM (.EXE)???
I MENTION THAT THE SOURCE FILE OF THIS
PROGRAM HAS BEEN LOST FOR EVER AND I MUST
MODIFY THIS PROGRAM ASAP !
IF SOMEONE WAS IN SUCH SITUATION , IS KINDLY
ASKED TO CONTACT ME ASAP AT THIS ADDRESS:
DOLJ@HOTMAIL.COM .
ANY SOLUTION TO SOLVE IT IS WELCOME !
THANK YOU IN ADVANCE !
EMIL