iPod help, anyone?

[ninjamonkeyspy]

I don't have an iPod and haven't used an iPod. I have barely used iTunes on my own, so my exposure to the hardware or software is very limited. However, my sister has a dilemma, and I'm putting my tech-savvy friends to the test to help her. Here goes:I had a bunch of mp3 files on my hard drive, and I imported them into my iTunes. So I then had them in two locations. So when I needed to free up space on my hard drive, I deleted the original files, knowing that I still had them duplicated in my iTunes library. Unfortunately, I evidently should've converted them to AAC first, because now they won't play and I'm getting a message that says the files are lost.

So what I need to know is how to recover them (I've emptied my Recycle Bin). And/or, how does one restore one's computer to a previous day/time, and, would that restore the lost files? I've done it before, but I can't figure out how to now.

Any advice would be most appreciated.

Also, for all you iTunes aficionados: because I don't want to wipe out anything that's currently on my iPod that's no longer on my computer, how do I change the settings from sync to manual without first plugging in my iPod and erasing stuff?? Is there no other way to change the settings BEFORE plugging my iPod into my computer? And/or is there a way to get music from my iPod BACK onto my computer?

Thanks for any and all help anyone could offer!