You can try setting the filters to Linear or Anisotropic to get the gpu to antialias or set it to point to basically tell it not to sample surrounding pixels.
// at the class level define a state
SamplerState ss_ansiostropic_clamp = new SamplerState()
Filter = TextureFilter.Anisotropic,
AddressU = TextureAddressMode.Clamp,
AddressV = TextureAddressMode.Clamp,
AddressW = TextureAddressMode.Clamp,
// in draw set it to the graphics device or pass it to begin.
GraphicsDevice.SamplerStates = ss_ansiostropic_clamp;
if your doing this on a shader you can set the minification and magnification filters directly to your texture sampler if your using mip maping set it to the mip filter.
sampler2D TextureSamplerA = sampler_state
Texture = <TextureA>;
//AddressU = Linear;//Mirror;//Linear;//Point;//Wrap;//
//AddressV = = Linear;//Mirror;//Linear;//Point;//Wrap;
//MinFilter = Anisotropic;//Linear;
//MagFilter = Anisotropic;//Linear;
//MipFilter = Point;
I think you should be able to set it up as well in the shaders technique were you define vs ps.