Nick pointed me to Ed Bott's posts on ZDNet.com about some reasons why Vista appears so slow "out of the box". Ed took a brand-new Sony Viao and then spent an age removing the crapware that Sony put on the machine, and updating all the drivers (essentially by doing a fresh install of Vista). The result? A zippy machine. In Ed's words "...Vista doesn’t suck".
Looking at the ZDNet website, it seems that Ed has also included a number of other posts to help people "improve" their Vista experience.