how to create tabbed blocks for recent comment and recent content? i want to create a tabbed block with two tabs . One for showing recent content and one for showing recent comments .. how can it be done in drupal 7 using only the modules ??
Use the quicktabs module: 

http://drupal.org/project/quicktabs

Install it normally, and then 'add quicktabs instance'. In the interface, select the two views you have that you want to display and add them to each of the tabs. Your new block will show up in the block menu to be placed using the blocks menu or other layout solution.