MySQL vs SQL Server Express (Compare Free DBMS)
I will be totally honest with you. While MySQL works fine in Windows, I would not recommend it for any Enterprise use in Windows.

You should go with SQL Server Express because you will have a database that works great in Windows for Free. Should your company fully commit to Enterprise Database usage, you could simply upgrade to SQL Server (different advanced versions).

If you are going for moderate budget, go with MySQL, BUT SWITCH TO Linux. High Availability solutions fit Linux and Unix, not Windows.