I came to US in 2003 August on F1 status. I was on F1 until Aug 2008 and I got H1B on October 2008.

I worked on CPT from January 2008 to Aug 2008 and on H1B from Oct 2008 to Dec 2008.

For the fist five years (2003 to 2007), I filed 1040 NREZ and form 8843.

Can I file form 1040 for 2008 and also form 843 to claim Social Security and Medicare taxes that were withheld during my CPT?

Thanks,
Ravi.

In 2008, you will file resident tax return.
Your CPT income is subject to FICA taxes. You will not get back SS tax and Medicare Tax. Read about F1-H1-B tax filing: Your U.S. Tax Return: The U.S. Visas (http://taxipay.blogspot.com/2008/08/us-visas.html)