First your SSD has to actually support TRIM for Windows 7 be able to use it, look up the specs of your SSD if you aren't sure if your SSD supports it.
How to check if TRIM is turned on in Windows 7
1) First open the command prompt with admin privlages
2) Type fsutil behavior query DisableDeleteNotify
It will either give you back a 1 or a 0. This is a little backwards to what you are use to. 1 is turned off and 0 is turned on.
If windows says "DisableDeleteNotify = 0" That means TRIM is enabled. If it says 1 then it's not enabled
How to enable TRIM on Windows 7 if your drive supports it and if the above command indicated it's currently off
1) Open the command prompt with admin prilvates
2) fsutil behavior set DisableNotify 0
TRIM should now be enabled on your SSD. Not sure if you have to reboot or not, but it never hurts.