Hoping to establish a new beachhead in the office automation marketplace, American Telephone & Telegraph Co. will introduce a new high-power personal computer and low-cost personal computer network next week.

The two products are AT&T's most aggressive effort yet to establish itself as an alternative to IBM's popular personal computers and proposed local area network system for desktop computers.

While AT&T currently offers a personal computer, the 6300, that is compatible with IBM's popular machine, the new 7300 relies on AT&T's own UNIX operating system and a powerful computer chip similar to the one found in Apple's Macintosh computer. The machine will come with 512 kilobytes of memory and is expandable.

The machine reportedly is easier to use than the 6300, relying on a Macintosh-like approach that allows the user to point to symbols on the computer screen that represent the desired function. It will be priced between $5,000 and $6,000.

The company will simultaneously introduce Starlan -- for Star Local Area Network -- a cable system that links personal computers in an office for high-speed communications. Local area networks allow office workers to transfer data to each other. AT&T is expected to position Starlan as its low-cost network blending data and voice communications. Up to 30 computers could be linked to Starlan, according to one source.