If you're writing a WPF application the chances are that you'll run across this little oddity fairly soon. On some monitors UIElements can appear slightly blurry or over anti-aliased, almost as if ClearType were over-compensating. The solution is to set the SnapsToDevicePixels property on the root element in your Visual Tree (all child element will then inherit this property).
Below the Username textbox with SnapsToDevicePixels set to false, N.B. the border is blurred:
Below the Username textbox with SnapsToDevicePixels set to true, N.B. the border is now crisp:
Seema Ramchandani has a great post explaining Pixel Snapping in more detail.