[J7N]

You have poor resolution video and are drawing the subtitle directly on this picture. When the video is later upsampled for display in fullscreen subs are processed in the same way (because now they're part of the video). On your WinXP machine subs are drawn over the video and always output 1:1 pixels without upsampling (if you want you can control the font size in pt).

There is an option in preferences to change between the two modes: Draw subtitles to overlay surface. I don't know if it has an effect in Vista.