Whether to reinstall or to attempt a clean up is a difficult choice. If a PC has been used previously, a reinstall is more or less essential because you don't know what is lurking. For a reinstall on a new PC, assume that it will take a minimum of three or four hours for XP, less for Vista. A couple of hours will be required for the base OS and a couple of further hours for drivers and essential apps. The length of time is primarily dependent on how easy it is to navigate the manufacturer's website for drivers. For an Intel desktop motherboard, it takes just twenty minutes to download all of the latest drivers, but other manufacturers make it much more difficult.
Wiping out the crapware on a new PC takes a similar length of time, so it is my preferred solution for an out-of-box PC.