I have shifted by Dev Env recently to Eclipse Helios on Windows 7. After that the Ctrl + Space Content Assist feature is not working. I found out that that key option is utilize
I use eclipse indigo and had this problem. Creating a new workspace did not worked.
Go to Windows->preference->General->Keys->"Restore Default" - This fixed my issue
Note that I just had English(US) on windows 7 [Control Panel-> Clock,Language and Region ->Change keyboard or other input methods-> Keyboards and Languages->change Keyboards...->General ==>Default language is English and Installed Services I just had English(US) alone
After doing this some you might need to restart the system if it does not fix.