The statute of limitations for refunds of taxes from the IRS is, generally, three years from the original (extended) due date of the return or two years from the date of payment. The return for 2001 was due 4/15/02 or, at the latest, 10/15/02 (if extended). Therefore the statute expired with respect to that year on 4/15/05 (or 10/15/05, if applicable) UNLESS you wound up paying the tax later than 10/15/03.
For example, if you filed the return 4/15/02 and owed $10,000 but did not pay the $10,000 until 4/15/04, you would still have until 4/15/06 to get a refund of up to $10,000. However, if you did not file a return (or did not file on time) but were due a refund because of withholding or estimated taxes paid, you will not be able to receive that refund.
To ensure compliance with requirements imposed by the IRS, you are hereby informed that any U.S. federal tax advice contained in this communication (including any attachments) is not intended or written to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing or recommending to another party any transaction or matter addressed herein.