If you ever find yourself with this error:

“Unable to find script library '/aspnet_client/system_web/1_1_4322/WebUIValidation.js'. Try placing this file manually, or reinstall by running 'aspnet_regiis -c'.”

and you've already ran aspnet_regiis but it still shows the error, try this:

“This problem most often occurs when you are also running virtual directory and the site you are hitting has a virtual directory redirect set for it. To fix this, copy the aspnet_client folder structure from the root of your account into the website 's subfolder (as listed in the Vitrual Director mappings).“

[taken from here.]

It certainly saved the day for me!