Why does my .net application require full trust?

I've developed a .net 3.0 application, which is deployed using clickonce.

I'd like to move from full trust to partial trust to ease deployment.

I've tried the "Calculate Permissions" tool in the "Security" tab of my project under visual studio, and the answer is quite clear :

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Microsoft Visual Studio
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This application requires full trust to run correctly.

However, I've not been able to figure out why full trust is required. I've tried to change the security settings to "partial trust", but the application raises a SecurityException immediately upon launch :

System.Security.SecurityException   {"Request failed.", Action= "System.Security.Permissions.SecurityAction.LinkDemand"
   at MyNameSpace.Program.Main(String[] args)
   at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
   at System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
   at System.Runtime.Hosting.ManifestRunner.Run(Boolean checkAptModel)
   at System.Runtime.Hosting.ManifestRunner.ExecuteAsAssembly()
   at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext, String[] activationCustomData)
   at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext)
   at System.Activator.CreateInstance(ActivationContext activationContext)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext e