ITunes keeps "losing" the filepaths to my library.

Question

My friend had an external hard drive on her computer, and every time she disconnected it, iTunes would lose her songs. So we copied all of her music to her hard drive, and put it in her iTunes folder. Everything was fine; all the file paths were correct, and she could successfully update her iPod and play the music in her iTunes. This was Monday night.
Today, she turned on her iTunes, and there were exclamation marks next to more than half of her songs. The other night, when she clicked on the first file to "locate" it on the computer for iTunes, she could then just click on the next file with an exclamation mark, and iTunes would find that file immediately (meaning, she wouldn't have to guide iTunes through each folder to show it where the music file was). But today, when she clicks on the exclamation marks, she has to guide iTunes to each individual file.
Why did iTunes lose the paths in the first place? We haven't moved any of the music files. And is there anything we can do short of reassigning the file paths for each individual song? Because I don't want to do that for 1000 songs, and then tomorrow, have a bunch of exclamation marks again. For the record, she's using iTunes version 7 (which she JUST put on her computer in the last month or so).

Answer 1

Close all Apps and see if Activity Monitor shows any processes runnung.
Not that I think this is the cure but have you tried these resets:
SMC RESET
http://support.apple.com/kb/HT3964
Shut down the computer.
Unplug the computer's power cord and ALL peripherals.
Wait 15 seconds.
Attach the computers power cable.
Wait another 5 seconds and press the power button to turn on the computer.
The 5 second timing is important to a successful reset.
PRAM RESET
Zap the PRAM
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.