Computer partition space problem

After updating the Windows XP to Windows 7, my computer becomes really slow and recently often receives a low disk space message. And there is only 3GB of free space left on my system partition. So, I think it is the time to extend my system partition space. But, I am not so familiar about the partition expanding things and don’t know what to do. Do you have any idea for me? Please, give me some suggestions! Thanks!