Why do textures sometimes look messed up? - Video Tutorial
Glitchy textures can sometimes be indicative of a problem with files in with your Second Lifeî viewer's cache. Try the steps indicated in our article on clearing your cache. Updating your graphics card drivers can also help in some cases.
In case the problem isn't Second Life-specific (such as hardware failure), running a stress test such as the ones available at the following links may help isolate the problem:
You can also try rebaking textures, as explained in this handy video tutorial: