$objWMIService = GetObject("winmgmts:\\"+ $workstation + "\root\cimv2").Get("Win32_Product")

$errReturn = $objWMIService.Install("c:\blahblah\testapp.msi", , ALL_USERS)

Produces this error :
Win32_Product
ERROR : expected ')'!
Script: C:\Temp\scripts\testinst.kix
Line : 5

Line5 -- > $errReturn = $objWMIService.Install("c:\blahblah\testapp.msi", , ALL_USERS)

Any ideas?