Ambika, welcome to Tom's Hardware Forums!
From the specs http://www.dell.com/in/p/inspiron-15-intel-n5050/fs
you have a good laptop. The Processor (CPU) and 4 GBs of RAM should be quite capable of running videos without any hitch.
It is possible that too many programs are loaded in memory during startup. These programs will use memory and if that is the case, the computer will tend to be sluggish.
Look at the "Startup folder" and uncheck (disable, not delete) programs that you are not going to use on a constant basis. If you are unfamiliar with this, ask a friend to help you do this. During startup only the bare minimum programs must be loaded. This will speed up your computer.