BigBoy (I think) meant to put REM at the beginning of the line, i.e. "rem C:\PROGRA~1\GRISOFT\AVGFRE~1\BOOTUP.EXE".
After changing that, there are two easy ways to get rid of the message all together I can think of, either:
a) Just delete the whole line, you don't need it.
b) Put "@ECHO OFF" at the top of autoexec.bat, if it's not already there. That will hide the command being run (so it won't display "rem C:\PROGRA~1\GRISOFT\AVGFRE~1\BOOTUP.EXE" on screen).