Not sure how to fix the silverlight issue. You might try clearing your temp internet files and try again. Also make sure you're an admin when you install. Check out the logitech forums for more ideas. I've seen the silverlight issue come up a lot over there.
Harmony is worth the effort, but IMO the 300 is not. Even when you get it working, it will do only one activity macro and has no LCD for additional functions. For a bit more money, a 600 (especially an open box or refurb) gives you a lot more functionality. I picked up an open box 600 from tiger direct the other day for around $35. The 300 is ok if you only watch cable or sat, but pretty worthless in a home theater. Even in that simple case, a standard $15 universal remote like the RCA RCRP05B has far more functionality than a 300.