Not exactly a true "Free Trial" as you describe is available. What folks that say that are probably thinking about is you can install it from the DVD and have a 30 day "grace period" prior to entering your product key. You may (well, probably will) get various reminders that you need to do so during that period, but the OS will work. Once the 30 days are up, you'll need to enter the product key to continue using the OS or remove it and start over with something else. I'm going to do this
There is a trick that should allow you to extend the 30 day period an additional two times. Microsoft's answer to your question is below (including the trick I mentioned to extend the deadline).
There is a 90 day free trial for the Enterprise Version of Windows 7. There are issues you should be aware of if you use it though. First off, the Enterprise version is more expensive and has strings attached (mostly the enterprise software agreement and minimum purchases you must agree to). Second, once the 90 day trial is up the OS stops working and you must wipe it out and install from scratch. Since this trial is designed for large businesses to test the OS out and not run it for real, there are no provisions to allow you to get anything off the OS after the test expires. The link below refers to this trial as well.
Can you download a Windows 7 Home Premium trial? - Microsoft Community
I second the comment to get the EOM version of the OS. It's the same as the retail version but is less expensive (ok, the packaging is different, but who cares about that). You must buy some hardware to qualify to buy it, but building a new PC I suspect you've done that. A few places do allow you to download the installation DVD instead of getting a physical copy so if you want the free trial only to save shipping time, this may be a way to accomplish that. The link below is one place that has a download delivery system (shop for others if you have a preferred company to shop with).
Windows 7 Home Premium - Microsoft Windows 7 - Microsoft Windows