I just went through this with a new dell, where I used the local windoze tools to make a usb backup imagine with their software to a 16gb drive, and just tossed it in a drawer if I ever need to restore things. That supposedly restores their recovery partitions and windoze itself, which should be everything...
I'd not put faith in some tier 1 rep telling you linux voids the warranty. It was probably one of those "What is linux? Yeah, don't do that." sort of comments ignorant agents might spew, but otherwise shouldn't matter. If you restore the disk and bios to prior function, they should be none the wiser anyways.
-mb