I just discovered that by changing the Start -> Control Panel -> Power Options in Vista Enterprise (running under build 3150), I was able to reduce the CPU consumption from 37-40% to about 17-20%. By default, the Power Options setting was Balanced, but I changed it to Power Saver, and CPU usage went down quite a bit. You might also reduce it further by changing the search indexer behavior under the Advanced Settings when you Edit Plan Settings.
I can confirm that setting the energy settings to Powersaver and disabling any unneeded services brought my parallels cpu from 120% to around 15%
Yes, I can confirm that this works well. My control panel is set up slightly differently so to get to the option described by feuerri I had to go Start -> Control Panel -> System and Maintenance -> Power Options.