If you have AVG .msi installer file, you could create .bat file and use msi parameters for linked AVG uninstallation and SEP installation.
For example, you have AVG's .msi installation file somewhere on your network;
Create .bat file which creates temp-folder to your computers, copies AVG's .msi installer file and SEP setup.exe to client's temp-folder, uninstalls AVG, and launches SEP installation.
I know that it might not be supported way to uninstall competitive AV software and after that install SEP without first rebooting computer, but that's something we usually do because we dont want that AV uninstallation runs fine, end-user shuts down his/her computer and leaves the office, goes home and starts computer without any AV/Firewall installed.
For AVG uninstallation and SEP installation, you might try something like this, assuming you have AVG's msi installer. Create text file, edit it;
md C:\Temp\Avinstall
xcopy \\yourserver\avg\installer\*.* C:\Temp\Avinstall
xcopy \\yourserver\folderwheresepclientsetupfileis\setup.exe C:\Temp\Avinstall
msiexec /x C:\Temp\Avinstall\avg.msi /q /REBOOT=Reallysuppress
C:\Temp\Avinstall\setup.exe
rd C:\Temp\Avinstall /s /q
^^Rename that text file to install.bat or whatever you like.
md creates Avinstall folder under C:\Temp
xcopy lines copies needed files for AVG uninstallation and SEP installation
msiexec /x uninstalls AVG using msi file you copied from your network share
setup.exe installs SEP client, of course you first need to create client installation package from SEP Manager
rd command removes Avinstall folder you created from C:\Temp
Test install.bat file first locally with few computers, if it works deploy it using Altiris/SMS/SCCM if you have one.
Cant really say if that works, but you might get the idea what to look for. If there's no script files in SEP CD2 for uninstalling competitive product, you need to figure it out how to uninstall it, there's usually quite many ways to do it. Here's some example for AVG, found it from web;
ECHO Check whether AVG is installed
CD /d "%ProgramFiles%\Grisoft\AVG Anti-Spyware 7.5"
IF NOT EXIST Uninstall.exe GOTO END
START /wait Uninstall.exe /S
ECHO Uninstall applet started
:END
ECHO AVG is not installed
^^ Might wanna add SEP Client setup to that script, you dont want to have situation where computer leaves office without AV/FW installed.