I was looking for last couple weeks for a good way to re-purpose PC’s as thin clients to ease the investment in VDI. I stumbled across this PDF from VMware and I thought it was great. I would tend towards using group policy to deploy the new shell described on pages 3 and 4. It can always be undone if the PC is needed as a PC again.
You pretty much replace the default shell (explorer.exe) with the VMware View Client. I would suggest using some group policy to keep people from using the task manager to start new processes. This should be a temporary solution until you have budget to buy some real thin clients or net books even.
There are of course lots of options out there for thin clients, and software to provision a “thin OS” to machines. This is free and easy though. I thought it was cool so I decided to share.