I believe if you insert the CS 2 install disk you can choose custom or to remove Illustrator there should be an option to uninstall you might have to run it twice once to remove what you have and one for what you had installed.
If you run it a second time and it does not give you a choice to remove anything then you are probably good to install. And it should run.
My brother has a legitimate copy of Illustrator CS2 that he doesn't use any more.
Assuming "doesn't use anymore" means he's upgraded to a newer version, so no longer need CS2..... that makes your install of Illustrator CS2 illegal.
The only way he can legally allow you to use the disk to install on your computer is if he completely uninstalls Illustrator. License transfer may be needed as well if you wish to be the legal owner of the application.
Not saying this is the case. Just pointing out the facts.
He is using CS4 at work. So he told me that I could use his personal CS2 that he bought years ago. Is that OK?
That is perfectly fine except you should transfer the license to you
and Adobe allows it. That way if you wish to
upgrade yourself you can do so.
You st`ill have to uninstall the trial probably using the disk. Then
you should be able to install.
Otherwise it mean an archival reinstall of the OS if you are on a Mac.