Compaq Computer Corp shook the computer sector yesterday with a warning that it expects sales for the second quarter to June 30 to be more than 15% below second quarter 1990 sales of $862m, which combined with the continued impact of the strong US dollar will likely cause earnings per share to be under $0.25 against $1.18 a year ago. The shock at the degree of the slump is all the greater for the fact that it comes a full six weeks before the end of the period, making it clear that Compaq sees no hope of recovering the situation this quarter. Compaq’s shares fell $10.625 to $38.625.