Deerfield, Ill., Walgreen Co., the nation’s largest drug store chain, plans to build a new distribution facility in Windsor, Conn., at the New England Tradeport, a 600-acre planned industrial park. Walgreens has signed a letter of intent to purchase a 130-acre site in the park. Site work on the property is expected to begin in late spring, with a planned opening in 2008. The facility’s size will be about 700,000 sq. ft., with the capability for future expansion.