Msiexec is the tool you need. It has a -q parameter to let it install stuff "quiet":
/qn : Displays no user interface.
/qb : Displays a basic user
interface.
/qr : Displays a reduced user
interface with a modal dialog box
displayed at the end of the
installation.
/qf : Displays the full user
interface with a modal dialog box
displayed at the end.
/qn+ : Displays no user interface,
except for a modal dialog box
displayed at the end.
/qb+ : Displays a basic user
interface with a modal dialog box
displayed at the end.
/qb- : Displays a basic user
interface with no modal dialog boxes.
You can also provide named options (or "properties") at the end of the command line; which properties are supported depends on the package. For example, this command:
msiexec /qb /l* perl-log.txt /i ActivePerl.msi PERL_PATH=Yes PERL_EXT=Yes
...is how you install ActiveState Perl, instructing the MSI package to add Perl.exe to your PATH and to associate .pl files with it. (source)