I did this because my comp sucks but it prob also very useful for peeps with nice rigs. I use win xp pro not sure if step 2 is possible on vista. There are tons other posts, resources around but few people seem to do these 2 and they help a ton.
1. Use a commercial defrag program. All those bliz patches and mpq files clog up your harddrive like a dirtty toilet. The windows defrag is a piece of junk.
I'd recommend Perfectdisk PerfectDisk Blog: World of Warcraft -- defrag your MPQ files
Or o&o defrag O&O Software - Creating Solutions
2. Boot wow using a gaming profile instead of your default login, basically follow this awesome guide
Tweak XP for Gaming « Red Chaos’ PC Tech & Gaming