If you are on Vista/Win2008 or up do this:
all at one line, I added linebreaks here for readability:
netsh advfirewall firewall add rule
name="SQL Server Windows NT"
dir=in
action=allow
program="C:\Program Files (x86)\Microsoft SQL Server\MSSQL.1\MSSQL\Binn\sqlservr.exe "
enable=yes
the kb-article detailing the new command.