Eu nunca vi ShowBalloonTip
usado dessa maneira. Isso deve funcionar:
[reflection.assembly]::loadwithpartialname("System.Windows.Forms")
[reflection.assembly]::loadwithpartialname("System.Drawing")
# notify.icon type: Information, Warning or Error.
$notify = new-object system.windows.forms.notifyicon
$notify.icon = [System.Drawing.SystemIcons]::Information
$notify.visible = $true
$notify.BalloonTipIcon = "Warning"
$notify.BalloonTipText = "The CPU is hot."
$notify.BalloonTipTitle = "Title"
$notify.ShowBalloonTip(1000)