Use PowerShell to stop 'Default Web Site' in IIS6
Check the serverState property. A value of 2 means running and a value of 3 means stopped, you can set them this way:
start the website
$site.serverState = 2
$site.setInfo()
stop the website
$site.serverState = 3
$site.setInfo()
http://www.microsoft.com/technet/scriptcenter/scripts/default.mspx?mfr=true. The example is in VBScript, but it's exactly the same technique in PowerShell.