The US Patent and Trademark Office (USPTO) has agreed to reopen the investigation into an Eolas patent that covers interactive web content. 

USPTO will give Microsoft an opportunity to prove that it invented interactive web technology before Eolas. If the Redmond company is proved right, the Eolas patent will be invalidated.

"We are pleased that the USPTO has decided to give us the opportunity to establish that we are the original inventors," said Andy Culbert, Microsoft associate general counsel, in an emailed statement.

"We look forward to establishing that our claims are valid and that the Eolas patent is invalid."

Microsoft has been engaged in a seven-year legal battle with Eolas and the University of California. A jury found in 2003 that Microsoft infringed on the patent and ordered it to pay $521m in damages.

Microsoft has appealed the ruling and attempted to get the patent invalidated. A retrial is scheduled for 9 July, but a ruling in the re-examination could take up to a year.

The disputed patent describes a way to embed interactive elements in documents, which includes applications embedded on websites.

Microsoft released a patch in March 2006 that circumvents the patent by disabling all embedded applications until the user clicks on them or presses a key.  

The changes did not affect the previous lawsuit, preventing Microsoft from infringing on the patent in the future.