I can help you out on the OS side. I hope you can help me out on the car side. That's where I'm a noob.
Your partitioning method is a good idea. Set aside a small system partition and the rest for media. That way, if and when your OS craps out you can just reformat that partition and not loose or need to back your data. Heck, if you are not putting too much applications with your OS 15 gigs may be too big and you can actually make your media partition bigger. It is Windows and Office that takes up a majority of the space along with the patches. I have 15 gigs reserved for my System drive on my gaming rig and I still have 40% free space. If you are going to run a stripped down Windows XP with a few apps like GPS than you can make your 15 gig smaller.
Now if you want to turn off unnecessary services that hog up system resources try looking up blackviper on the internet. I use this chart to disable many uneeded services. Unfortunately for some reason his site has been down for ages: http://blackviper.com/
Here it is archived: