If you have an iPhone or iPod Touch....Intelliphone does exactly this.

Intelliremote Client for iPhone/iPod Touch is a native iPhone application that allows you to control your home theatre PC running Windows. Since the iPhone uses a network connection it does not need "line-of-sight" like traditional infrared remotes and you can use it anywhere in or out of your house! Intelliphone also acts as a wireless keyboard and mouse so you can get rid of those in your living room HTPC setup and just use your iPhone since you always have that on you!

Wireless Keyboard and Mouse like a Logitech Dinovo Mini? YEP.

[Link: melloware.com]