NUnit assembly not found

I've used NUnit before, but not in a while, and never on this machine. I unzipped version 2.4.8 under Program Files, and I keep getting this error when trying to load my tests.

Could not load file or assembly 'nunit.framework, Version=2.4.8.0, Culture=neutral, PublicKeyToken=96d09a1eb7f44a77' or one of its dependencies. The system cannot find the file specified**

In order to simplify the problem, I've compiled the most basic possible test file.

using NUnit.Framework;

namespace test
{
    [TestFixture]
    public class Tester
    {
        [Test]
        public void ATest()
        {
            Assert.IsTrue(false, "At least the test ran!");
        }
    }
}

I've added "C:\Program Files\NUnit-2.4.8-net-2.0\bin" to my PATH (and rebooted). Note that if I copy the test assembly into that folder, then

C:\Program Files\NUnit-2.4.8-net-2.0\bin>nunit-console test.dll

works, but

C:\Program Files\NUnit-2.4.8-net-2.0\bin>nunit-console c:\dev\nunit_test\test.dll

and

C:\dev\nunit_test>nunit_console test.dll

fail with the above error.

Presumably I could get around this by copying the NUnit.Framework DLL file into my project's bin folder, but I don't remember having to do this in the past. Moreover, I get the same error in the GUI. Shouldn't the GUI know where the framework is located (that is, in the same folder)?

I'm not using Visual Studio. I use the following line to compile the test project.

%windir%\Microsoft.NET\Framework\v2.0.50727\csc.exe /r:"C:\Program Files\NUnit-2.4.8-net-2.0\bin\nunit.framework.dll" /t:library /out:test.dll test.cs

I tried both the .msi and the .zip file with the same result.