'Think about' means you keep something always on your mind for a period of time.
e.g. I was thinking about buying a new car if I got promoted at work.
'Think of' is something slightly different. 'Think of' has a meaning of just remembering what happened or what you have done, as well as who you met.. e.g. A worker may ask his manager if the manager remembers the emails that were sent him yesterday. The worker can say:" Mr Smith, can you think of where those emails are, that you sent me yesterday?"