Hamilton Mortgage and Investment Co. Inc. has opened an office in Lanham, Md. Lee A. Horner, president, said that the firm's fifth office was the third in Maryland to be opened in the six-year history of the firm. He said the mortgages being serviced on residences now totaled $180 million. Thomas L. Vernon was named manager of the Lanham office.

For information about new real estate courses being offered by the Arlington Distributive Education Service in conjunction with the Northern Virginia Board of Realtors, Call 558-2581.

Isadore Mostow, who heads an investment firm in Silver Spring, announced that he has purchased the 14-year-old, 50-unit Slige Hills apartments on Manchester Road in Silver Spring for about $400,000. Silgo Hills Corp., headed by Vincent Vinella, was the seller.

A conference on new ideas in neighborhood preservation will be held Thursday and Friday at the National Housing Center, 15th and M streets NW, by the D.C. Department of Housing & Community Development in cooperation with the Washington Board of Realtors, the D.C. Home Builders Association, the Potomac chapter of the National Association of Housing and Redevelopment Officials and the Metropolitan Washington Savings and Loan League. For information, call 724-8789.

Garth Marston, chairman of the Federal Home Lean Bank Board, will be the speaker at noon Monday when the Mortgage Bankers Association of Metropolitan Washington meets at the Mayflower.

Lawrence McGuire, a partner in Merkli-McGuire Realtors in Alexandria, has been awarded the IFA designation by the National Association of Independent Fee Appraisers. Members are full-time professional real estate appraisers who qualify by means of experience and examination of ability.

Eric Bauer was the top producer of Century 21 Kadow Inc., realtors, in Bethesda with a total of $650,000 in gross residential sales in February. In real estate selling five months, he is taking the Century 21 sales training program.

Real estate courses that prepare persons to take the broker and selling licenses in Virginia will be offered at the Annandale campus of Northern Virginia Community College. For information, call 323-3160.

The third national conference on community associations will be held March 27-30 at the Continental Plaza Hotel in Chicago. For information, call the Community Associations Institute based in the District.