Can I stream video from PC to an android device over USB? I'd like to be able to stream video to my android phone (Samsung Galaxy S rooted 2.2) from a PC running Windows XP (I'm at work!).

There's no wireless network here, VLC and similar marketplace apps seem to only offer this service over wifi/3G; can I over USB?
You could turn on "USB Tethering", then run some media server like VLC carefully configured with RTSP streaming on your local server. Then open up the stock browser on android, type in your local ip (prefixed with rtsp:// protocol instead of http) of the pc followed by the port number as set in VLC or equivalent. The Android media player should launch if all is set and should start to play the video.

The local ip must be the ip of the PC running VLC or equivalent media server. You'll need to find it through right clicking the active LAN connection (of usb tether) > status > "IP address : "..

Do disable firewalls or add exclusions appropriately for this to work, also, set the VLC with proper video transcoding format that your android can play.

Warning: you might want to disable data pack when enabling the usb tether to avoid incurring unwanted costs.