These instructions are not mine, they are 76dragon's (TPB) and I take no credit for them in any way. They are being used because they worked for me without any incident or hiccup!!!

Part 1
REMOVE OLD ADOBE INSTALLATIONS (IF APPLICABLE) BEFORE INSTALLING ADOBE CS5
Remove the following Adobe applications and files before installing the Adobe CS5 Master Collection:
/Applications/Adobe? (everything with Adobe)
/Library/Application Support/Adobe
/Library/Application Support/FLEXnet Publisher
/Library/Preferences/com.adobe... (everything with com.adobe)
~/Library/Application Support/Adobe
~/Library/Application Support/Adobe/Acrobat
~/Library/Preferences/Acrobat
~/Library/Preferences/Acrobat Distiller Prefs
~/Library/Preferences/Adobe... (everything with Adobe)
~/Library/Preferences/com.adobe... (everything with com.adobe)

Part 2
INSTALLATION OF ADOBE Illustrator CS5 (Or any other CS5 pruduct)
Disconnect from network/internet (Turn AirPort Off). Double click "Adobe CS5 Master Collection.dmg" to install the Adobe CS5 Master Collection. Whilst installing CS5, ensure that you choose "Install this product as a trial" and DO NOT enter any serials. If you've already installed CS5 applications using a serial, load any of the installed CS5 applications and select Help > Deactivate. Quit all of the installed CS5 applications.

TERMINAL
Launch a Terminal window (in /Applications/Utilities) and copy and paste: sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /etc/hosts

Hit return and type your admin password.
The "hosts" file should automatically open in TextEdit.

TEXTEDIT
If the following lines are already part of the "hosts" file close it and continue with the "Finish" section. If not hit return (in the opened "hosts" file) and copy and paste following lines:
127.0.0.1 activate.adobe.com
127.0.0.1 practivate.adobe.com
127.0.0.1 ereg.adobe.com
127.0.0.1 activate.wip3.adobe.com
127.0.0.1 wip3.adobe.com
127.0.0.1