BartPE, because sometimes you actually have to run Windows.
If you have Windows and your system did not come with a "rescue", "restore", or "emergency startup" type CD (yes, I'm talking to the cheap bastards at Dell who don't include the Windows CD), then you NEED to follow the instructions at http://www.howtohaven.com/system/live-windows-rescue-cd.shtml. This creates a bootable "BartPE" CD with all the right utilities for fixin' what ails ye.
Do it now! Before you need it, not after. I learned the hard way, and after this past week, I can't sing the praises of BartPE enough. It was a life saver.
My wife's laptop booted to a BSOD when I was trying to VPN in to work. Oh well, a night off....but then it kept doing it, over and over. I couldn't get Windows to start for the life of me.
Since I didn't have a bootable Windows CD, I was left trying to boot FreeDOS from my thumbdrive (using LiveUSB Creator) . I made sure to have a copy of NTFS4DOS installed on there as well, and ran through some exercises where I got a command prompt to manually copy registry files around. That was painful, but it actually worked enough to get the machine to boot into XP. But it was a version of XP that had all sorts of issues, error messages, drivers crashing, no USB support, no CD recording, no network, etc. Without USB, network, or CD burning, I couldn't get our important files off the machine.
Anyway, to make a long story short, the FreeDOS/ntfs4dos route was crappy and painful with no real reward.
The BartPE rocks the house! It boots into a minimal XP interface with all the right utilities for fixing your issues, and USB drives are recognized, so I can copy important files off the machine in case I feel the need to wipe it clean.
I guess I really should do regular backups, but what fun is that? Preventing problems is boring, solving problems is heroic....