In its latest trading statement, group like-for-like gross margins were down 0.6%, in line with trends experienced in the first quarter, it said. But the margin reduction was as high as 2% in the PC World computing division.
DSGi said this was "largely driven by slower Vista-related hardware sales and a changing sales mix in computing".
Sir John Collins, DSGi chairman, said, "PC World delivered good sales performance against a tough prior year comparative in the back to school period. The reduction in laptop stocks that arose out of disappointing sales of Vista related products, and a changing sales mix, have reduced gross margins by around 2% in the computing division."
This has reduced group profits by around £20m in the first half, said Collins.
"Stocks are now at normal levels and we expect to recover some of the lost margin through the second half," he said.
The group will announce interim results for the 24 weeks to 13 October on 28 November.