Buildout : Operating System

Since all of the modern VSTs support Windows 7, that's what I went with. The particular PC I bought, didn't have a CD/DVD drive, so I used PenDriveLinux (http://www.pendrivelinux.com/) to put the Windows installation on a USB stick. Next, I booted to BIOS and set it up to boot to USB.

After installing Windows, I tweaked it a little. I disabled the following services :
Background Intelligent Transfer
Offline Files
Print Spooler
Windows Update (if you're going to keep this thing plugged into a network, you'll want to manually update periodically.... problem with leaving this service running is that it'll want to download, install and reboot your PC all the time... even when you don't want it to). UPDATE: In order to use Update manually, you'll need to temporarily enable this service in order to do so.
Windows Search
Windows Media Player Network Sharing

I disabled the screensaver and under Control Panel->System->Adjust the appearance and performance of Windows, I selected "Adjust for best performance".

NOTE: I had installed Microsoft Security Essentials, but found that it added another 15 seconds to boot time. It's a gamble, but I uninstalled it. I won't be browsing the Internet or installing any questionable software on this PC.

Since I don't have keyboard/video/mouse attached to the PC, I've assigned the power button to "shut down". This is under Control Panel->Hardware and Sound->Power Options->System Settings

I also disabled all of the system sounds. This is in the Control Panel and is called "change system sounds". Set the Sound Scheme to "No Sounds" and uncheck "Play Windows Startup Sound"

Disable SuperFetch service.  In a live performance you're only going to be loading 1 application : your VST host.  You don't need Windows spending time trying to figure out and pre-load applications it thinks you might like to open at some point.

Disable ReadyBoot : This service knocks off 20% of your boot time; however, keeps the hard drive thrashing for 30-40 seconds after boot.  Definitely not worth it... especially if you're loading tons of sounds into memory at boot up.  To disable, launch the "Reliability and Performance Monitor" program (under "System Tools" inside "Computer Management"; or via any of the other routes to launch this program). Then on the lefthand side of your screen click "Data Collector Sets", and underneath that heading click "Startup Event Trace Sessions". Now on your righthand side you'll see a list that includes ReadyBoot, and you'll see the word 'enabled' beside the word 'Readyboot'. Double clicking the list item brings up the ReadyBoot Properties dialog. This dialog has a number of tabs. Pick the "Trace Session" tab. Finally, uncheck the "Enabled" checkbox on that tab.  IMPORTANT: you have to disable the SuperFetch server or else, Windows will just re-enable ReadyBoot next time the PC starts up.