There's a couple of possible solutions if you're experiencing signal strength issues.
Not enough signal
If you're using rabbit ears or a small antenna, like the kind included with most USB tuners, then you have to be pretty lucky to get a good signal. Digital TV is no different to analogue TV: roof-top antennas will always give the best results and are highly recommended.
Too much signal
If you're using a roof-top antenna, are close to a transmission tower and are getting a good signal on your TV or set top box, then there's a chance you're getting too much signal and you're overloading your media center or USB tuner. You'll need an antenna attenuator. These can be purchased from most electronics stores.