I struggled with this for a while. Finally found this as a take-off point: http://www.msfn.org/board/topic/14456-trillian-pro2/page__hl__trillian
Installed trillian-v18.104.22.168 on a clean test machine.
Copy the C:\Program Files (x86)\Trillian folder, add the shortcut, add it to a zip with the following batch file:
XCOPY *.* "C:\Program Files (x86)\Trillian" /E /C /I /Q /Y
REG ADD %KEY% /V "DisplayName" /D "Trillian" /F
REG ADD %KEY% /V "DisplayIcon" /D "C:\Program Files (x86)\Trillian\Trillian.exe" /F
REG ADD %KEY% /V "Publisher" /D "Cerulean Studios, LLC" /F
REG ADD %KEY% /V "UninstallString" /D "C:\\Program Files (x86)\\Trillian\\Trillian.exe /uninstall" /F
copy Trillian.lnk "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\"
Add postinstall app
Upload zip to K2000
Run batch file
Running it on a win7 x64 scripted install
seems to work fine