If you set the motherboard controller to AHCI mode before you installed Win7, and if that drive supports TRIM, TRIM will be enabled by default. Wait for someone with more experience than I have to come along and post the command that checks whether or not it is actually enabled in the OS, as a double-check.
Did you install Win7 with the controller in the default IDE mode?
EDIT: From here: http://forums.legitreviews.com/about23670.html
How To Make Sure Trim is Enabled in Windows 7
Postby Apoptosis » Sun Oct 25, 2009 8:34 pm
The Trim command should be enabled by default on Windows 7, but if you want to check to make sure do this:
Command prompt > fsutil behavior query disabledeletenotify
DisableDeleteNotify = 1 (Windows TRIM commands are disabled)
DisableDeleteNotify = 0 (Windows TRIM commands are enabled)