For some reason when I start up the game everything looks fine, but once I start playing, all of the text in the menu, as well as the text that appears at the bottom of the HUD at times, becomes unreadable and glitched.
Can anyone say why that happens? It doesn't hurt the gameplay really, but it's just a nigging issue to me.

Try the usual stuff first. Update your video drivers and DirectX.
Also, make sure you don't have any mods installed.