Function Get-FileName($initialDirectory)
{  
 [System.Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms") |
 Out-Null

 $OpenFileDialog = New-Object System.Windows.Forms.OpenFileDialog
 $OpenFileDialog.initialDirectory = $initialDirectory
 $OpenFileDialog.filter = "All files (*.*)| *.*"
 $OpenFileDialog.ShowDialog() | Out-Null
 $OpenFileDialog.filename
}

Get-FileName -initialDirectory "c:\fso"