Microsoft has been found guilty of wilful patent infringement, and has been ordered to pay $388m (£264m) in damages to anti-piracy software developer Uniloc.
Uniloc develops systems to prevent a single piece of software from being installed on multiple PCs, and sued Microsoft in 2003 claiming that Windows XP and other Office products infringed on some of its patents covering software activation.
In a case lasting nearly three weeks, a jury at the US District Court in Rhode Island sided in favour of Uniloc. The decision overturned a previous ruling that cleared Microsoft of the charges, following a US Court of Appeals decision to remand the matter back to the District Court for trial last August.
"They have been struggling for six years against Microsoft the giant, and finally justice has prevailed," said Paul Hayes, an intellectual property patent attorney at Mintz Levin, which represented Uniloc.
The payout is thought to one of the largest on record for patent related cases.
"We are very disappointed in the jury verdict. We believe that we do not infringe, that the patent is invalid and that this award of damages is legally and factually unsupported," said Microsoft in a statement.
Microsoft said that it plans to appeal the decision.
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