Whether you want a light OS to use on unfamiliar PCs, or just want to experience a Linux distro, Puppy is a good choice for a flash drive based system.
Step 1: ISO Recorder
First, you need to burn Puppy's live CD. If you're using Windows XP and don't have advanced CD creation tools, I recommend downloading ISORecorder.
Once you've installed it, you will find a new option "Burn CD Image" in your right-click context menu.
Step 2: Burn CD
Step 3: Change Boot Sequence
Step 4: Reboot Into Linux
Your PC will now load Puppy's live CD; it will prompt you to define your mouse, keyboard, and video settings. These menus are fairly self explanatory, just follow them and you should be fine.
Step 5: Universal Installer
In the main menu, navigate to Setup>Puppy Universal Installer.
The installer will take you through the steps of installation. When prompted to choose how to install [partition, etc], pick the first option [install to present partition].
Step 6: Partition
Step 7: End Linux Session
When Puppy has completed installing to the drive, the CD will likely pop out and you'll be prompted to save your session- choose no.
Step 8: Reboot & Change Boot Sequence Back
Step 9: USB Key Complete
You should now boot into Puppy automatically upon restart.
There's no need to enter setup each time you wish to switch to Windows; simply reboot without the USB key plugged in, and the system will default to Windows.
Step 10: Resources & Tips
Note that some pen tablets aren't recognized right off the bat- you may want to favor a standard mouse for the setup. If your mouse doesn't work, F12 will pop up the main menu, and ALT + hotkey will allow you to navigate menus.
ISO recorder can be found here: http://isorecorder.alexfeinman.com/isorecorder.htm
The Linux distro can be found here: http://puppylinux.org/main/index.php?file=Overview%20and%20Getting%20Started.htm