Here's a quick example of code to see if installed.
code:
;  check if already installed
$SC3uninstall = ReadValue("HKEY_LOCAL_MACHINE\software\microsoft\windows\currentversion\uninstall\ShowCase STRATEGY 3.0", "uninstallstring")
if $SC3uninstall = ""
  goto install
else
  goto finish
endif
If you tell us what software and what installer technology it uses, we could better advise on automating a silent install. FE, in the case of ShowCase, the following is what I did.

code:
$RC = MessageBox("Silent install will run now."
 + Chr(13) + "Please be patient!","ShowCase Strategy Install",64,60)
USE L: /DELETE
USE L: "\\FFSMS\SC3"
SHELL("L:\CLIENT\SETUP.EXE -S")
_________________________
Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.