If you're like me, chances are you use a version of Vista on your PC. Chances are that you don't use nearly all of the programs & features of Vista neither; so why bother having them installed in the first place?
If you want to dramatically increase Vista's performance, the best thing to do is reinstall the OS. But why install from the original disk...when you don't have to? Not too long ago, i've stumbled across a program called vLite. vLite lets users build a custom disk image (ISO) with as many or as few features as they want. So if you've been really wanting to get rid of the Sidebar or User Account Control, now's your chance! Prevent these nuisances from being installed so you can have more memory for what's really important.
If you are satisfied with Vista, don't bother. It involves a complete re-installation of the OS. However, if you've tried everything to increase Vista's performance (Error Checking, Defragmenting, Registry Error Checking, Running 3rd Party System Optimizers, Disabling Services, ReadyBoost, etc.) and just makes it worse, try vLite.
From my experience, I installed my custom Vista ISO on a virtual machine (for testing purposes), and so far, Vista runs with no errors, and features like Windows Update, iSCSI, Windows Sideshow, and UAC are gone for good!! They're not listed in the Control Panel. Not once have I received a UAC prompt.
However, there is a catch. In order to successfully use vLite, you'll need an original Vista installation disk, so it can extract files to the hard drive.
For more info on vLite, visit http://www.vlite.net/about.html