Okay, if you base your database for the system in SQL Server, you have a few options.

You can either use SQL Mail or send via SMTP directly using CDOSYS (see links below);

http://support.microsoft.com/kb/263556

http://support.microsoft.com/kb/312839

It really depends on what mail options you have on the client you'll be controlling the system from.

If you have access to an exchange server or can configure a connection via outlook, then SQL Mail can usually be the better option.

The rest is down to your database design, and how you want to capture "transactions."

If you're creating a "logging" type of system, with an insert into a table when someone enters a building via your voice recognition function, and want immediate notification sent to an admin, you can attach an insert trigger to the table (see below);

http://msdn.microsoft.com/en-us/library/ms189799.aspx

Alternatively, you can use a stored procedure and/or a sql script configured to run on a schedule using a SQL Server Agent job to fire out periodic alerts.

If you use this method, you'll need to put a procedure into place to take into account already notified records.

This should give you a start anyway.