Your friend's PC probably doesn't boot from a flash drive by default, so you'll have to figure out how to make it do that. When the computer first comes on, it may produce messages telling you what keys to press for 'Setup' or the 'Boot Menu'. If it offers both, go with the Boot Menu option, since Setup will require you to change the boot order of your friend's machine (you should change it back when you're through).

If you see no such messages, experiment with pressing function keys just before Windows loads (F10 is a good bet for the Boot Menu). You can also search the Web for setup or "boot menu" plus the PC make and model number.

Even if you know the appropriate button, getting the timing right can be tricky. I just push and release the button over and over again until the correct menu comes up.

(If you're wondering why you shouldn't simply create a bootable CD instead of a flash drive, I can list several reasons: You can't easily copy files to a CD. Not all PCs have optical drives anymore. And not all computers with optical drives boot from them without the same hassles I just described.)