Well, your hard disk is full. There's two solutions to this:
1) Get more hard disk.
2) Delete crap.
The hardware solution is to get another drive. This can be done two ways:
1) Replace the hard drive in the cube with a huge one
2) Add an external FireWire 400 drive. (The Cube does not have USB 2, so an external USB drive would be horribly slow)
Replacing the drive isn't TOO hard compared to other computers. If you have a nearby Mac geek you can bribe, they can do it in under an hour including time to copy all the files over. A PC geek won't have any trouble changing the drive out, but might need some clues on how to copy the drive over. Here's what I use to do that: http://www.bombich.com/software/ccc.html
Adding an external hard drive is easier, but it has the downside of having a cable come out, etc etc. You can either scan dealmac.com for a pre-built external drive, or you can buy a drive case and a bare drive. The current going rate for hard drives is under $1 per gigabyte, and a FireWire 400 case is about $30 to $50.
That's hardware. Here's software:
There's a superb disk utility called Disk Inventory X which you can get here http://www.derlien.com/
(I hope. The website seems to be down at the moment.)
Anyway, you run it, and you'll see a lovely picture full of rectangles. The big rectangles are large files. Often I find a few of those I didn't know were there, and they can be deleted. Note that files in the "Unix" directories, such as /usr, /var, /bin, /sbin. and /private should be left alone.
Next to the picture, there's a list of directories. The ones at the top are the space hogs. Click on one and you'll see a rectangular outline showing how big it is. Looking on my disk, the largest space hog is in fact my music folder. But maybe you'll find something you copied once, and forgot you had. A movie? A huge game that you played once and hate??
I bet you see /Library/Packages on that list. Everything in there is stuff that software update downloaded and ran, but for some reason never deleted. I bet there's a few gigabytes there.
Another software utility of love is called OnyX http://www.titanium.free.fr/pgs2/english/onyx_tiger.html
Click the "Automation" tab in the middle, and check all the boxes except "Display of folders content", make sure you don't have any unsaved documents, and let it rip. OnyX will delete all the cached files, some of which are probably 5 years old and obsolete for 4.99 years. Onyx will also "Repair Disk Permissions" and "Verify Disk."
At the end, OnyX will want to restart your computer, and that's why you don't want to be working on anything important. The other thing is that although all the stuff it cleans up is cached or otherwise unimportant, it will take a while to re-index for Spotlight (if you have 10.4), and to re-download cached email stuff. Run it at night, then restart, then log in, then run Mail and hit "Synchronize Local Accounts" and let it take care of itself overnight.
The other thing to do is to get rid of applications and their associated junk. If you don't use GarageBand, you can get rid of about 2.5 or so gigs of stuff. iDVD is good for a whole lot, too.
GarageBand has an .app in /Applications, and /Library/Application Support/GarageBand and maybe /Library/Audio/Apple Loops/ can also be deleted.
iDVD has an .app and maybe /Library/Application Support/iDVD.
In general, there's a shareware called AppZapper that helps delete all the extra crud that apps have. I haven't used it, but lots of people swear by it.