Tax demand and refund

I expected a refund for IT Return for FY 2010-2011 but IT Department adjusted it against some demand raised against FY 2002-2003 & 2007-2008 though I had received refunds for FY 2004-05 & FY 2005-06. How this can happen? If there was a demand  for FY 2002-2003 it  could be raised or adjusted against refunds for FY 2004-2005 & FY 2005-06? if still needed a demand could be raised?. Till how many years IT Department can raise demand? Thanking you in advance. RAKESH