Here is how you repair/fix/check your disk for errors, *even* if MacOS X is on your only partition (that's what's going on on my computer).

Go to System Preferences and choose Startup Disk.
wait. wait. wait. wait. wait. wait some more.
You will see the option to boot MacOS 9 from the same drive the MacOS X is.
Make sure you click lock icon and enter root for user and whatever your root password is.
The available options will look something like this:

Macintosh HD (Mac OS X)

System Folder
Macintosh HD (Classic MacOS 9)

Select the Classic Mac OS 9.
Press lock to lock the changes.
Close System Preferences.
Log out.

Wohooo! The MacOS 9 will be booting.
Put your MacOS X Install CD in your CD-ROM.

Go to Utilities.
Run Disk First Aid.
You can choose repair. Wohoo!
It will warn you to close all the apps.
Let it do that.
Let it fix all errors.

Cool. Wanna go back to MacOS 9?
Choose System Disk.
Select MacOS X.
Close System Disk.

Welcome back.
Your disk is fixed and you can run MacOS X happily.
Actually if you want to get unix-y there is a much simpler way. When you get a crash or restart, immediately after you restart hold down the "s" key.

After you see the mac icon it will go to a black screen with white text, then let go of the "s" key. It will print alot of text and finally tell you it is entering single user mode and fsck has not been run. Type this:

/sbin/fsck -y

Hit return and wait. You will be text like in Disk First Aid and if it finds an error will tell you it's repairing the disk. Once it's all finished, type:

shutdown -r now

The machine will reboot and you'll be back in Mac OS X. You should run the Disk Utility to check the disk has been repaired, but everything should be fine.