First thing I'd do is make a backup of her data if she hasn't already.
Then I'd try booting to safe mode (hold shift while booting the computer up). This will check the disk for errors, and also eliminate any extra startup programs. If it works OK in Safe Mode, reboot and see if normal mode is OK - the disk check could have fixed it, OR there could be some other piece of software causing you problems.
But if it still works poorly in safe mode, the next thing I'd check is the hard drive - especially if it's more than a couple years old. You can run Apple's built-in hardware test by holding "D" when your mac boots, although this isn't the most reliable test in my experience.
If none of that turns up anything, I'd look into more thorough Hard Drive diagnostics - The Apple Store can do this for you, or you can use something like the
ultimate boot CD.
