Install-WindowsFeature -name Web-Server -IncludeManagementTools

if([Environment]::Is64BitProcess)
{
    Install-WindowsFeature -name Web-Server -IncludeManagementTools;
}else{
    Start-Process -FilePath "C:WindowsSystem32WindowsPowerShellv1.0powershell.exe"
    //это директория с х64 версией
}