Here are some pictures of the textures: http://img713.imageshack.us/img713/4764/screenshot0wn.png http://img248.imageshack.us/img248/8641/screenshot2qn.png
I don't have any mods enabled. What could cause this problem and how could I fix it?

I take it you're on PC, so let's get this thread moved to the hardware/software forum for better assistance. In the meantime, run a DxDiag and post the results.

What kind of graphic card do you have? It might help if you try different graphic card drivers.

This is probably not the solution, however it's easy enough to do. Reinstall the shaders: Oblivion Shaders: http://www.tesnexus.com/downloads/file.php?id=20348

My graphics card is nVidia GeForce FX 5200 (yes, it's old). Passed all the DxDiag tests. I reinstalled the shaders; it made no difference.

FX 5200 is woefully inadequate to the task. You really need a better vid card. My guess though, is that you have an older system, probably a single core processor, and only AGP slot to work with...... might be time to look at buying a new computer.