If your WordPress plugin supports an ancient version of Piwik (around June 2010/v0.6.2), and you don't mind exposing the security vulnerabilities fixed since then (for example, if deploying on a company intranet), there is a patch adding support for SQL Server 2008 that was rejected.
Since then database access has been re-architected, so the patch is unlikely to be easy to merge into the current version of the software. There has been some interest in adding support for SQL Server:
Open Tickets
#2593: Multiple database and storage engine support 2011-07-27
#1902: Piwik on Windows: SQL Server, Azure, Wincache 2010-12-10
Forum Posts
Piwik using MS SQL 2012-11-15
Microsoft SQL Server 2008 2010-07-30
Microsoft Sql Server 2010-04-20
And here is a list of Windows-friendly alternatives, though they don't seem to promise easy WordPress integration:
http://stackoverflow.com/questions/1164936
Since then database access has been re-architected, so the patch is unlikely to be easy to merge into the current version of the software. There has been some interest in adding support for SQL Server:
Open Tickets
#2593: Multiple database and storage engine support 2011-07-27
#1902: Piwik on Windows: SQL Server, Azure, Wincache 2010-12-10
Forum Posts
Piwik using MS SQL 2012-11-15
Microsoft SQL Server 2008 2010-07-30
Microsoft Sql Server 2010-04-20
And here is a list of Windows-friendly alternatives, though they don't seem to promise easy WordPress integration:
http://stackoverflow.com/questions/1164936