I'm having the same problem as everyone else. The SQL 2008 Express installer tries to install Powershell, even though Powershell 1.0 is already installed.
Then, I'm informed that Powershell installation failed, and that SQL 2008 Express installer failed too, because Powershell was a dependency.
I've gone in and uninstalled Powershell, and tried everything again. I even went to $NtUninstallKB926139$ and uninstalled Powershell manually. This failed on both my home and work computers.
I'm stymied.