You could always boot straight into a Windows install CD and blow away the vendor's OS install entirely. The problem with doing that, is that often (particularly on laptops) you need a bunch of special purpose drivers to get everything working. Finding and downloading these can be a hassle.
I doubt this is true anymore. Centrino is well-standardized (and what other laptop platform is there?), and Windows 7 is not as outdated as XP with respect to hardware support. Maybe if you saved fifty cents by getting "Dell Wireless" instead of the Intel chipset, you'll have problems... but I don't know anyone who does that. (Even Dell's site tries to talk you out of it.)
I have never used a machine with the default OS install on it, and I have never had any hardware support problems. (And this includes my XP machines at work. Definitely not the default HP Windows install, but the hardware works fine nonetheless.)
