Trouble Installing Photoshop CS3

I had to uninstall Photoshop cs3 about a month ago and installed cs4. My trial for cs4 just ended so I uninstalled that and tried to reinstall cs3. Everytime I trial to install it it keeps on stopping the process and telling me to insert the disk for cs4. I don't understand why it isn't letting me install the old version.

Is there anything I can do. I did the CS4 Cleanup Utilility and nothing showed up. I really don't want to have to upgrade to cs4 when I have the disk for cs3. Should I try to install it to a external hard drive and try to drag it into my laptop?

Thanks for any help.

It can be a real pain to fully extract CS4, but a quick Google search should pull up the places to look to eliminate ALL traces of CS4, at which point you will be allowed to reinstall CS3. ISTR there's even a page on Adobe's site that details how to fully uninstall the trial of CS4.