I think you can open your website project file such as Alloy.csproject in your editor such as notepad++ or others. And seach for <UsingTask> node and if you find it, then uncomment the node or just delete it and save your project file. Then you can rebuild your solution correctly.
I have installed a brand new installation of EPiServer 7. Installed the Demo site. Opened the demo site in VS2012. When I build it, the following error is thrown. I have VS2012 installed on the machine. After some googling I have also installed Team Explorer for MS VS2012 and Visual Studio Express 2012 Web in hopes to find the right build dlls. I have Microsoft.Web.Publishing.Tasks.dll and Microsoft.WebApplication.targets in C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v10.0\WebApplications and C:\Windows\Microsoft.NET\Framework\v4.0.30319. What I can do so the demo site compiles?
Error 2 The "TransformXml" task was not found. Check the following: 1.) The name of the task in the project file is the same as the name of the task class. 2.) The task class is "public" and implements the Microsoft.Build.Framework.ITask interface. 3.) The task is correctly declared with <UsingTask> in the project file, or in the *.tasks files located in the "C:\Windows\Microsoft.NET\Framework\v4.0.30319" directory. C:\EPiServer\Sites\MyEPiServerSite\[MSBuild]\CommonConfigTransforms.xml 20 5 Alloy