Microsoft recommends that you choose a password that will be required when starting Windows 7 before access to your user account will be allowed.
Don't treat this as a recommendation - consider it a requirement.
In the Type a password (recommended): text box, enter a complicated but easy-for-YOU-to-remember password. Retype the same password in the Retype your password: text box.
Type a hint to give yourself in the Type a password hint (required): text box. This hint will display if you enter the wrong password when logging on to Windows 7.
As you can see in the example above, the hint I entered was What is my favorite food?. The password I entered (which you can't see above) was applesauce.
Note: Feel free to use the same password as you used in the operating system you've just removed from your computer as part of this Windows 7 clean install. However, this is as good a time as any to choose a stronger password than you might have used before.