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!
If you want to access the desktop of your Windows machine(XP or Vista) from N800, the best option would be to use RDP and rDesktop for N800 is an application that lets you do exactly that.