Hewlett-Packard says \"higher than expected\" failures of some desktop PC motherboards led it to offer a one-year warranty extension to cover possible repairs. In an e-mail to owners Tuesday, HP said numerous models in its Pavilion line of desktop computers \"have system boards with a slightly higher than expected failure rate,\" Computerworld reported. Pavilion Elite HPE-4xx, Pavilion P66xx and Pavilion Slimline S5660f desktops built in June and July 2010 \"may experience no video at start up and stop responding,\" HP said in the e-mail. The computer maker has not recalled the machines, but has instead added 12 months to existing warranty plans for the PCs. HP said it will repair systems by replacing motherboards free of charge and will pay for round-trip shipping between the user and HP repair station. Only a \"small percentage of systems in the serial number range\" have either already experienced or may experience a motherboard failure, the company said.