Microsoft has delivered its last service pack for Office 2007.

Service Pack 3 (SP3), which weighs in at 351MB, was posted to Microsoft's download center yesterday, and has begun appearing as an optional install through Windows Update.

Microsoft won't start automatically downloading and installing Office 2007 SP3 onto customers' PCs for 90 days.

SP3 is Office's 2007's first service pack since 2009's SP2 , and with the upcoming end of mainstream support, almost certainly its last.

The service pack's debut was not a surprise: Earlier this month, Microsoft promised to ship the update this month.

Microsoft was under a self-imposed deadline to get SP3 out in October because it wanted to give enterprise customers six months to test and deploy before the suite is retired from mainstream support on April 10, 2012.

Once Office 2007 exits mainstream support, Microsoft will supply most customers security updates only during a five-year extended support phrase that runs through April 11, 2017.

Users can continue to run Office 2007 SP2 -- that edition doesn't stop working -- but Microsoft will halt all security updates to SP2 a year from now.

Links to the download pages for the various permutations of Office 2007 SP3 can be found in this Microsoft blog post.

Office 2007 went on general sale in January 2007 alongside Windows Vista, but was supplanted by Office 2010 more than a year ago.