proc DllEntryPoint hinstDLL,fdwReason,lpvReserved
     mov     eax, TRUE
     ret
endp

proc AVX
     push ebx
     mov eax,1
     cpuid
     and ecx,18000000h
     cmp ecx,18000000h
     jne no_AVX
     xor ecx,ecx
     xgetbv
     and eax,110b
     cmp eax,110b
     jne no_AVX
     mov eax, 2
     pop ebx
     ret
no_AVX:
     mov eax, 1
     pop ebx
     ret
endp    

    public partial class Form1 : Form
    {
        [DllImport(@"C:\FASM\DLL\AVX\AVX.DLL")]
        private static extern uint AVX();

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (AVX() == 1)
                textBox1.Text = "AVX Support: No";
            else if (AVX() == 2)
                textBox1.Text = "AVX Support: Yes";
            else 
                textBox1.Text = "AVX Support: Unknown";
        }
    }    

The three main registers you can change:
EAX: the return value
ECX: don't care
EDX: don't care

All others you must not change: EDI, ESI, EBP, EBX, ESP return with the same value they had before.

But do take note that you can use your own calling convention if your DLL is used only by your code. The above only applies to things like the C (c-call) and Windows (standard call) calling conventions.