First: Which flavor of Ubuntu
First, figure out which flavor of Ubuntu will work well with your old computer. It may be too old for the standard Ubuntu. See How do I find out which version and derivative of Ubuntu is right for my hardware in terms of minimal system requirements? for guidance on this subject.
I recommend that you use a current version of (some flavor of) Ubuntu. Older versions of Ubuntu have reached the end of their life such as version 9.04 are not supported in this site.
One official flavor of Ubuntu is Ubuntu Mate,(See: https://ubuntu-mate.org). If the current versions 18.04 LTS or 19.04 of Ubuntu Mate is suitable for your old computer, then use this version, and flavor. The reason is, assuming you used the plain vanilla Ubuntu 9.04 back when you used this old computer, this flavor of Ubuntu is closest to the old Ubuntu 9.04 you were used to.
Second: Download and Create Live USB
Once you have figured out which flavor, download the most recent version of the ISO file from the official Ubuntu flavor web page using a different computer.
Using that other computer create a Live USB following the instructions in the official Ubuntu web page. This process is a bit different from copying the ISO file (or dragging and dropping it) to the USB drive.
Third: Try without Installing
Boot the old computer using the Live USB drive you created. Select the option to Try Ubuntu without Installing. This will boot the new version of Ubuntu from the USB drive. Try it out and see if it works on your old computer. If it does, go to the next step
Fourth: Copy the important stuff
Now is the time to look for a backup drive. An external USB hard drive may be best.
Explore the internal hard drive of the old computer using the file management software such as Nautilus. If your internal drive is mounted as /media/ubuntu/bunch.of.numbers.and.letters
then your personal files may be in:
/media/ubuntu/bunch.of.numbers.and.letters/home/username
where username
is the name you created when you (or whoever) installed Ubuntu in the old computer. If the old computer had multiple users, you may have to copy personal files from their respective folders.
Copy the folders containing your important files such as:
/media/ubuntu/bunch.of.numbers.and.letters/username/Documents
/media/ubuntu/bunch.of.numbers.and.letters/username/Pictures
/media/ubuntu/bunch.of.numbers.and.letters/username/Music
/media/ubuntu/bunch.of.numbers.and.letters/username/Videos
, etc. into the backup external USB hard drive.
See this answer for more explanation: I want to copy my HOME folder to a USB flash drive but I can't. I accidentally removed Python 3 and lost many important stuff
Hope this helps.