Unable to find script library '/aspnet_client/system_web/1_1_4322/web UI Validation.js'

Error encountered: Unable to find script library '/aspnet_client/system_web/1_1_4322/web UI Validation.js' Try placing this file manually or reinstall by
running 'aspnet_regiis.c'.

I created a new website in IIS, not a new virtual directory under the default website. It seems that non-default websites could not access the aspnet_client in wwwroot. Therefore just copy the aspnet_client folder from C:\inetpub\wwwroot then paste it inside the folder of the new website. Voila!