Dell inspiron 620s motherboard

It's usually the Super I/O controller for low-bandwidth devices - i.e., keyboard, etc.

My CPU isn't booting up and i think the cause will be the failure of SMSC chip. What should i do now?

If that chip has failed (it's not clear why you think it has, and it's not a common point of failure) all you can do is replace the mainboard.

If you haven't tried a system reset (uplug system, hold power button for 30 sec, reconnect and power up again), that's something you should do.

Do you mean the whole motherboard?

Yes. Except for the CPU (if it's socketed) nothing else is replaceable short of replacing the board.

So what should I do now? Should I replace the motherboard by sending it to Dell company? But my warranty period has gone. I purchased the PC in November 2011.

The boards cost about $130

accessories.us.dell.com/.../productdetail.aspx

It's your call as to whether to replace it yourself (it's not that difficult to do), or have a shop do the job - if you do the latter, look for a local shop that charges by the hour for labor rather than by the job.   An hour's labor might be $75 or so depending on your location.

Then again, $200+ is over half the cost of a newer system.  Weigh that in your decision.