Launcher loads, but does not display pop-up for log-in--it just says "Welcome, guest! Please log in. Ready to download & play Minecraft 1.6.4" The old launcher worked fine, before the horse update. I've tried reinstalling the appdata and Java multiple times.

Here's what seems to be wrong in the Development Console:

Unhandled exception in executor net.minecraft.launcher.updater.ExceptionalThreadPoolExecutor$ExceptionalFutureTask@cbe1b0d
java.util.concurrent.ExecutionException: java.lang.NoSuchMethodError: org.apache.commons.io.FileUtils.writeStringToFile(Ljava/io/File;Ljava/lang/String;)V
at java.util.concurrent.FutureTask.report(Unknown Source)
at java.util.concurrent.FutureTask.get(Unknown Source)
at net.minecraft.launcher.updater.ExceptionalThreadPoolExecutor$ExceptionalFutureTask.done(ExceptionalThreadPoolExecutor.java:54)
at java.util.concurrent.FutureTask.finishCompletion(Unknown Source)
at java.util.concurrent.FutureTask.setException(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoSuchMethodError: org.apache.commons.io.FileUtils.writeStringToFile(Ljava/io/File;Ljava/lang/String;)V
at net.minecraft.launcher.profile.ProfileManager.saveProfiles(ProfileManager.java:49)
at net.minecraft.launcher.Launcher.showLoginPrompt(Launcher.java:231)
at net.minecraft.launcher.Launcher.ensureLoggedIn(Launcher.java:192)
at net.minecraft.launcher.Launcher$2.run(Launcher.java:182)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
... 4 more