Three options:
(1) Try adding this line:
GraphicsDevice.SamplerStates[0] = SamplerState.LinearClamp;
This may change the appearance of the texture.
(2)
Change the height and width of the texture such that height * width is a power of 2. (i.e. ((2^9) * (2^9)) = 512*512 = 2^18)
(3) Change the XNA profile from Reach to Hi-def.
Right-click your project in Solution Explorer
Choose Properties
Focus the XNA Game Studio tab, and make your selection
(http://blogs.msdn.com/b/shawnhar/archive/2010/07/19/selecting-reach-vs-hidef.aspx)