I understand you're looking to control without seeing the actual pc screen, so only the apple remove actually does that... but the other two are great for full keyboard and mouse replacement since you have an iphone and a car PC might as well replace the big input devices, which are difficult to use in the car!
Air mouse, Rowmote Pro and Apple Remote are all awesome apps I use everyday to control my Media PC in my living room (actually a Mac Mini, but I think all these aps work on pc as well).
All 3 apps use wifi and thus the iphone must be connected to the same wireless network as the computer you want to control!
Air Mouse: (Mac and PC compatible)
awesome mouse and application control with full qrty keyboard and programmable hot keys.
Rowmote Pro: (Mac compatible only)
Awesome media application control (has a full mouse and keyboard as well, but it's remote mode that makes the screen look like an apple remote
Apple Remote: (Mac and PC compatible with iTunes only)
works great to control iTunes library without even being in the same room (don't have to look at the PC monitor). you control the PC's media library as if it was your local iphone / ipod touch's library!
Hope that helps! :)
i second the "Air Mouse" app. It works perfect and easy to use.