Hi,
I was on H4 from 2006 Dec and moved to F1 in March 2009. I am on F1 OPT from Nov 2011 to Nov 2012. My husband is on H1. We have 1 kid. I need help on completing my W4. We are planning to file jointly. How many allowances do I have to claim ? Please help. Also am I eligible for social and medicare tax exempt ?

You are exempt from calculating days for the substantial presence test for 5 calendar years on F1.
That would make you a nonresident alien. If you want to file jointly, you would have to add a statement the first time you do to make the choice to be treated as a resident alien.
As a resident alien you are not exempt from social security and medicare.
If you choose to be treated as a resident alien, you can use he IRS withholding calculator.
Note that it's your choice how many allowances you claim, just make sure you won't owe too much in the end (and your refund isn't too high which would be an interest free loan to the government)
http://www.irs.gov/individuals/article/0, id=96196,00.html
If you file as a nonresident alien, you cannot claim any allowances except for your own:
http://www.irs.gov/businesses/small/international/article/0, id=106274,00.html