Statute of Limitations

My question, is in regards to creditors Statute of Limitations.
I recently was contacted by a credit card company, who claimes they have bought my debt from another credit card company. They claim I owe them $3552.44, and are demanding payment.

This was a credit card debt. The state in which I opened the account (North Carolina) has a Statute of Limitations of 4 years. I opened the account in 1991. In 1996, while living in Arizona (Statute of Limitations in Arizona is 3 years), I ran into money troubles, and was unable to pay the debt.

The last time I made a payment to this creditor, was August 19, 1996 (while living in Arizona). From August 1996 to May of 1997, interest was added onto the debt. The current total of $3552.44, has remained the same since May, 1997.
In addition, this debt has been dealt with by several different collection agencies since 1997. However, the letter I recently received, is the first contact I have had from anyone regarding this debt, in over two years. Currently, I am living in Alabama (The Statute of Limitation in Alabama is 3 years).

My questions are:
1. Have the Statute of Limitations run out on this creditor?
2. When exactly does the Statute of Limitations start running?
3. How should I respond to the letter I recently received from the Credit Card company that has "bought" this debt?