The investment firm of computer mogul Michael Dell bought a majority ownership of the ValleyCrest Companies, the decades old family-owned landscaping firm based in Calabasas. Financial terms of the deal between ValleyCrest and MSD Capital L.P. were not disclosed. Company founder and chief executive Burton S. Sperber, President Richard A. Sperber and Stuart Sperber, chief executive of Valley Crest Tree Co. retain what the company called significant equity ownership and will continue to lead the business.