Quarterdeck Corp. said it is entering the market for anti-virus software, adding to the competition that has led to concern over high share prices for makers of anti-virus software. Quarterdeck, a company that is best known for software that allows users to manage memory on their personal computers, said it has acquired the worldwide rights to sell ViruSweep, an anti-virus program developed by EliaShim Ltd. The move by Marina del Rey-based Quarterdeck gives it a toehold in one of the fastest-growing segments of the software industry, analysts said. Quarterdeck needed to broaden its product line after Microsoft Corp.'s Windows 95 ate into its established market, said Terrence McCrary, an analyst at Auerbach Pollak & Richardson. Quarterdeck's shares rose 6 cents to close at $2.69 on Nasdaq.