If you have the disk space and are trying to extract to an user area like desktop, then check the size of the download. You may have to download again as download may be corrupted.
Problem solved - SIMPLE and QUICK solution
I too had similar problems installing Illustrator CS5. It just wouldn't complete the installation. Everything else in the CS5 collection installed fine. I looked around on the internet. Assuming that most people's installation problems originate from the same cause—registry errors, then scanning the registry for errors and correcting them first before rerunning the install should solve the problem. Which is exactly what I did and it worked like a charm.
Once you figure out what the problem is, how to fix it and get the right tools, the rest is easy. This is what I did. It took only a matter of minutes:
I use CCleaner (stands for Crap Cleaner, it's top-notch freeware I've been using for years, I can't recommend it highly enough!) to run a registry scan first. Sure enough, it identified at least one error with Illustrator. I scanned and fixed the errors in a couple of mouse clicks, took seconds.
I then run the CS5 installer again for Illustrator. Miraculously, it installed without a hitch!
You can download CCleaner here: http://www.piriform.com/ccleaner/download (The download link is in the top right corner of the page).
Hope this will help you too.
As a bonus, you will find that CCleaner can do a lot of wonderful maintenance stuff for your PC!