While upgrading an XP installation to Vista, the Vista compatibility check informed me that Windows Powershell must be uninstalled before installation can proceed. Going into Add or Remove programs did not show any entry for Powershell.
Turns out Powershell installs as an update to windows. You have to click on "show updates", and scroll through tons of security fixes and hotfixes. You'll find an entry for Windows Powershell 1.0, and your Vista upgrade should then install after that.
