Well, you've got a couple of options so I'll lay them out for you.
1. Your best option would be to buy an external hard drive. They are pretty inexpensive nowadays, and you can easily get a TB (1,000 GBs) for about $100. You can even get these at a local Wal-Mart at you convenience. Two decent brands are Western Digital and Seagate.
2. Do a quick clean of your computer. To do this press the windows button + r (I'm assuming you have a PC) and type "cleanmgr". Or click start or the Vista button in the lower left corner, click on run and type "cleanmgr". You can also go to the "More Options" tab in the Disk Cleanup and remove unused programs.
3. To thoroughly clean your computer use Ccleaner. Think of it as a more advance version of Disk Cleanup. Here is the link for the download.
http://www.ccleaner.com/download
4. Turn off System Restore. This takes up A LOT of space in your hard drive. What system restore does is automatically create restore points in your hard drive, this take up a lot of space,but if you have your own back ups, it's best to turn it off. If you don't fully feel safe turning it off, turn it off temporarily, do what you need to do, and turn it back on. To turn it off or on go to your control panel, search "system restore" and turn it off.
5. Sweetheart, the days of using your computer as storage are over. The new hot thing is Cloud Storage: storing your files online. Try skydrive.live.com for 25GB of free online storage. There are other services similar to this one, but this is the only one I can think of off the top of my head. Don't pay for online storage!
6. Use Gmail as an online hard drive, and by this I don't mean e-mailing yourself. By using this program http://www.viksoe.dk/code/gmail.htm you can create a gmail folder on your computer. When you put stuff into this folder from your computer, it automatically saves it on Gmail's server, and off of your computer. OOoooh.
7. For a similar experience as the one mentioned in #6, I also recommend dropbox. Another, online storage endeavor that works well. Here's the link https://www.getdropbox.com/
8. It sounds like you need to defrag. This should clean up your disks a bit. To do this do windows button + r again, and type "defrag". Or go to the start or vista button and click run, type "defrag".
9. And finally. If music is what is hogging space on your computer, use lala.com. It will scan your itunes music, and save it online for you to hear anytime you want. After this, you can upload your music either online or to an external hard drive, but be able to listen to all your music as long as you've got a connection to the internet.
Hope this helps! :)