Or if you don't want to just buy a full copy of Windows, wipe the hard drive with something like KillDisk and then install the OS and drivers from scratch and get rid of all that preinstalled [censored]
I get my systems built as its just easier than messing about with deliveries and RMAs but I always do the above to get rid of the preinstalled image.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen Lite 8000
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.