I've connected my windows7 laptop to my TV using an HDMI cable so that I can watch iplayer, youtube, google videos etc. Is there any way I can control the video playback remotely using my iPhone 3. I'd like to be able to pause, play, rewind etc the video playing.
Yes, there are numerous apps in the iTunes app store that will let you do this.
Firstly you need to decide if you need a keyboard or just a mouse clicker? Remember that Windows 7 has an excellent on-screen keyboard available.
In the case then of just needing a mouse clicker, you may want to consider a remote touchpad app such as Touchpad from Splashtop.
If you need more, there are hundreds of apps to choose from. A selection are listed here
You could even go for a full desktop remote control tool. However, you should note that you will not be able to use Microsoft's built-in remote desktop (RDP) since that will close down the normal screen output. However, there are plenty of alternatives even here. You could use a VNC based remote screen or something like TeamViewer.