When I run my disk it says my os is not supported

Are you referring to the Windows defaults or something else?

You need to run the "System file checker". Below is instructions on how to do that. You MIGHT need your dell OS disk, should have shipped with your Dell. If you can't find it, any Windows OS disk that matches your existing OS will work.

http://support.microsoft.com/kb/929833

You're going to need a bootable disk that came with the machine (or a version of the Mac OS that you know will run on it). You should do a complete custom install, including erasing the original disk.

I think you're going to need Leopard (Mac OS X 10.5) or earlier to get started. You can look for a disk on the internet, or see if you can get one from Apple (try the support forums at apple.com). A G4 is a PowerPC-based machine and isn't supported by the recent OS X versions, which require an Intel-based processor.

The problem is your computer is not compatable to run the Ubuntu operational system, it comes up not supported. But you can go online to forums that deal with these types of problems, because there are a lot of people out there that swear by this system.

Try a safe boot.
Shutdown your machine. Hold down the shift key. Poweron. Wait awhile Wait awhile while you harddrive
is being checked.
http://support.apple.com/kb/ht1455
Could be a corrupt disk. Run Disk Utility.
I suspect disk corruptions.
verify & repair your startup drive
To verify & repair you file system on the startup drive, you will need to run disk utility from you installation DVD.
This article will tell you how to get to disk utility. Once in a disk utility, you can go and attempt to recover the disk.
http://support.apple.com/kb/TS1417

To repair your startup drive, you will need to run disk utility from your startup DVD.
Mac OS X 10.4: About the utilities available on the Mac OS X 10.4 Install DVD
http://support.apple.com/kb/HT2055

you will need either to \ A.) Buy New OS disks B.) Contact Customer support for your particular machine and order recovery disks.
There are others sources for each particular O.S. however, it is most likely Illegal to obtain these as they are most likely Pirated.
Please Keep in mind that you may be charged for Recovery disks Just because Self modifications .. such as dual booting and such are not usualy covered under the manufacturers warranty.
Windows 7 Compatibility Check http://www.microsoft.com/windows/windows-7/default.aspx
After Running a Compatibility Check you can dtermine whiuch windows product to install.

Google "Ubuntu" and follow the download and burn instructions.

start>accessories>disc cleanup
start>accessories>defrag
Run Virus checker
Start>My Computers>check how full your hard drive is, if over 50% copy/delete some data off of it
Start>run>type msconfig>this will show all of your start up programs, look and see if you can delete some..

any questions let me know..

1.Insert the Mac OS X Install CD or DVD.

2.Restart Mac, immediately after the bong, hold down the C key until the Apple logo appears.

3.When the Installer reaches the INTRODUCTION phase, click on Utilities at the top of the screen, and drop down to Disk Utility.

4.Click the First Aid tab.

5.Highlight the boot hard drive volume or partition.

6.Click Repair Disk.

7.Repeat steps 5 and 6 until the scan comes up all clean and good (and run it one more time).

8.Quit Disk Utility and the OS Installer and reboot.

9.Firewire Target Disk Mode is another option we should know about.

Cheers.

Unfortunately, you cannot download the OS. You can, however, download the drivers directly from the Gateway website.

Gateway usually sells restore disks after asking you some basic questions. The disk should run you ~$20. Go to http://support.gateway.com/support/ask_gateway.asp and click on "Chat with Technical Support" and tell them your situation and what you need.

Good luck!