Superfetch is a funky new service in Vista that tries to pre-cache all the files that you use on a regular basis to make your apps run faster and snappier. While I'm sure this is a good idea in general, if you make heavy use of VMs on your machine you may have noticed a rather heavy load on the HDD.
On my laptop I've narrowed this down to Superfetch trying to pre-cache all of my .vhd and .vmdk files into memory as they're the files that I access the most. Suffice to say a) they don't fit so it's a waste of time, and b) it does this at the expense of loading other apps. I guess it's logic is that I load Outlook once a week, but I nail the .vmdk files all day every day so that's what it spends the time loading.
Disabling Superfetch has transformed the system, and now the hard disk light goes out sometimes. Try it out if you're seeing similar issues.
As always, YMMV,
Neil.