Since I've been learning from you folks for some time, it's time to give something back.
Jaina, I'm assuming that you are using System 10.2 or later.
As to the question about iTunes, you can install a purchased song on five computers. iTunes reads the serial number of each computer, so putting it back on the same computer doesn't matter. Burning a playlist through iTunes is limited to seven times, although you can burn a song an unlimited number of times. Some of the songs you bought from iTunes can be updated to a higher quality AAC and DRM Free version for a nominal fee. Only certain labels have agreed to this.
Disk Utility is safe to use. Open the program and click on the First Aid Button. You should see two hard-drive icons. The lower indented icon is the volume. Choose that and then click on Verify Disk Permissions. If any permissions are corrupted, choose Repair Disk Permissions. Choose Verify Disk to check the integrity of the volume structure. It is a good idea to run Disk Utility First Aid before and after any update or installation. I generally use it about once a month.
The erase function in Disk Utility is for reformatting the drive. Don't do that. It doesn't erase the drive if you click on the "Erase" button next to "First Aid," only if if you choose the buttons inside.
Raid is for advanced formatting of multiple hard-drives. You'll probably never need that.
Restore is helpful if you have a clone of your drive and the system becomes so corrupt that you have to reinstall every program. Just clicking on the Restore button next to Raid just opens another window.
One reason a computer slows is because of disk fragmentation. When you install Apple Updates, the system is optimized but becomes fragmented after a while. Downloading and deleting files can cause this, but so can normal use.
Another reason for sluggishness is that the system creates log files for anything it does. One place to see these is to open Safari and choose Activity from the Window Menu. Another place to view these files is to open Console in the Utilities Folder. If they've never been deleted they're probably quite large.
The system also saves caches and temporary files in the Library folder. These also become very large over time. Font caches, not the fonts themselves, are notorious for becoming corrupt and causing various problems. When opening Word you might have noticed that it takes a few seconds to optimize the font menu. This is a temporary file that doesn't always get deleted.
The operating system deletes most of these files for you, but only if the computer is on and not in sleep mode between the hours of 3:15 and 5:30 AM every day. To perform this operation manually, open Terminal in the Utilities Folder and type in a string of Unix commands into the core of the operating system. Beware, typos can result in severe punishment. Not exactly a shining example of Apple's ease of use, and definitely not for the faint of heart.
The easiest way to clear these files and optimize the hard-drive is by using a maintenance program. I use ONYX because it is reliable and free. If you are running system 10.5, Apple has it on their site. http://www.apple.com/downloads/macosx/system_disk_utilities/onyx.html
If you are using 10.3 or 10.4 you can download and read reviews here:http://www.macupdate.com/info.php/id/11582/onyx
Apple also recommends these programs:http://www.apple.com/downloads/macosx/system_disk_utilities/macaroni.htmlhttp://www.apple.com/downloads/macosx/system_disk_utilities/cocktail.html
Randomly deleting all files can result in lost web-page passwords or other problems. If you feel comfortable using such a program, I'd be happy to help.