One of my standard practices when setting up a new Mac or installing a new version of OS X is to create a spare user account. A spare user account is just an administrator account that you set up but never use except when you need to troubleshoot problems with the Mac OS or applications.
The idea is to have a pristine user account with a set of untouched preference files. With such an account available, you can more easily diagnose problems with applications or OS X.
How to Use a Spare Account to Troubleshoot
When you're having problems with your Mac that aren't (or don't appear to be) hardware-related, such as an application always freezing or OS X stalling and displaying the dreaded rainbow cursor, chances are you have a corrupt preference file. That's the easy part; the tough question is, which preference file has gone bad? OS X and any applications you install have preference files located in multiple places. They can be found at /Library/Preferences, as well as in the user account location, which is /username/Library/Preferences.
The easiest way to identify the culprit is to log out of your normal user account and log back in using the spare user account. Once you log in, you'll be using an account that has clean, untouched preference files. If you were having trouble with an application, launch that application and see if the same problem occurs. If it doesn't, chances are the application's preference files at /username/Library/Preferences are corrupt. It's a simple matter of deleting those preferences to restore the application to working health.
The same is true for general OS X issues; try duplicating the events that cause problems. If you can't duplicate the event with the pristine spare user account, then the problem is in your normal user account's data, most likely a preference file.
If the application or OS problem still occurs when you're using the spare user account, then it's a system-wide issue, most likely one or more corrupt files in the /Library/Preferences location. It could also be an incompatibility with a system-wide service or an application you recently installed; even a bad system font may be the issue.
A spare user account is a troubleshooting tool that is easy to set up and always ready to use. It won't actually solve any problems you may have, but it can point you in the right direction.
Create a Spare User Account
I recommend creating a spare administrator account rather than a standard account. The administrator account gives you more flexibility, allowing you to access, copy, and delete files during the troubleshooting process.
The easiest way to create a spare administrator account is to follow the Add Administrator Accounts to Your Mac guide. This guide was written for the Leopard OS (OS X 10.5.x), but it will work just fine for Snow Leopard (10.6.x) as well.
You'll need to choose a username and password for the new account. Because you'll rarely or never actually use this account, it's important to pick a password that's easy to remember. It's also important to pick a password that isn't easy for someone else to guess, since an administrator account has an enhanced set of privileges. Although I don't normally recommend using the same password in multiple places, in this case, I think using the same password you use for your normal account is warranted. After all, the last thing you want when you're trying to troubleshoot a problem is to be stuck because you can't remember a password you created a long time ago for an account you hardly ever use.