How do I create a bootable USB Flash PcRepair Kit?

PenDriveLinux is a spectacular option. It creates everything you need for the bootable USB drive to repair pc's. You can boot up to a linux build of your choice, or to a repair disk of your choice (i.e. Ultimate Boot CD). Pendrivelinux also has Gparted, DBAN, Avira rescue disk and many other options. It even has an option to setup a windows installation (it says vista/7 but I'm pretty sure you could get an xp install on it too) If you're looking for an all in one, PenDriveLinux multiboot is what your looking for.

I always use Hiren's boot CD. It have lots of tools in a very nice and organized menu.

There's a very detailed tutorial on how make this boot CD into a USB Flash drive. link.

The one I've used most frequently is BARTPE. It's a little bit clunky, but can be expanded to include your own tools and generally does the trick. In particular, it lets me access a PC's hard drive and files without having to boot from that drive, thereby avoiding any startup issues, malware, lost passwords, and so on. It automatically tries to connect to available networks, allowing access to web-based tools and antivirus updates.