When you install an Adobe application or Creative Suite, you are prompted for a serial number, which is checked against Adobe’s activation server (if the installer you are using requires activation, not all do).
Adobe terminated their CS2 activation server on 12/13/12, so if you have a copy of Adobe Creative Suite 2 that requires activation and try to re-install it, you will get an error that the serial number can’t be validated.
To address this issue, Adobe has posted serials numbers for CS2 apps (both Mac and Windows), plus links to download the installers, here: http://www.adobe.com/downloads/cs2_downloads/index.html
Although there’s nothing stopping anyone from downloading and use these serial numbers, Adobe has posted to their forums “On behalf of Adobe Systems Incorporated… You have heard wrong! Adobe is absolutely not providing free copies of CS2! You are only legally entitled to download and install with that serial number if you have a valid license to the product!”.