I don't think powershell is a good choice for automated installation because first you have to install powershell (unless this is windows 2008 R2) and then you have to enable script execution before your scripts will run.
Instead us pkgmgr or ocsetup which can be run from a regular cmd.exe console.
This should work for IIS 7 (Windows 2008)
start /w pkgmgr /iu:IIS-WebServerManagementTools;IIS-ManagementConsole;
start /w pkgmgr /iu:IIS-ManagementScriptingTools;IIS-ManagementService;
start /w pkgmgr /iu:IIS-IIS6ManagementCompatibility;IIS-Metabase;
If you are really determined to use powershell this will install it on Windows 2008
start /w pkgmgr /iu:MicrosoftWindowsPowerShell
Keep in mind that this installs the Windows 2008 OS "Feature" and so you will get PowerShell version 1.0 not PowerShell version 2.0.
This article explains how to get PowerShell v 2 for your OS (2008 and earlier)
http://support.microsoft.com/kb/968929