Apple TV and Air Parrot Software would be a relatively cheap option.
Basically you connect up the Apple TV onto the Television and set it up to be connected to your wifi.
On the PC's and your Macs install Air Parrot. If you are using a Mac you can even set it up so that the tv is an extension of your desktop (acts like a 2nd monitor). Sadly on the PC, Air Parrot only supports mirroring of your desktop (still fine for presentations). Of course your PC and Macs will have to be connected to your work network either by wireless or wired connection.
Hope this helps.
Last time i checked the Apple TV 3rd gens are £100 and Air Parrot is a not expensive.
You can then run presentations, powerpoints on computers and have them appear on the screen. Movies should work fine too. Air Parrot supports sound as well.