GUID guid = {0};
CoCreateGuid(&guid);
dwRand = guid.Data1 & 0x7FFFFFFF;

Data 1 - unsigned long
Data 2 - unsigned short
Data 3 - unsigned short
Data 4 - unsigned char

public struct MyGuid
{
    public int Data1;
    public short Data2;
    public short Data3;
    public byte[] Data4;

    public MyGuid(Guid g)
    {
        byte[] gBytes = g.ToByteArray();
        Data1 = BitConverter.ToInt32(gBytes, 0);
        Data2 = BitConverter.ToInt16(gBytes, 4);
        Data3 = BitConverter.ToInt16(gBytes, 6);
        Data4 = new Byte[8];
        Buffer.BlockCopy(gBytes, 8, Data4, 0, 8);
    }

    public Guid ToGuid()
    {
        return new Guid(Data1, Data2, Data3, Data4);
    }
}

Guid g = GetGuidFromSomewhere();
MyGuid mg = new MyGuid(g);
mg.Data1 &= 0x7FFFFFFF;
g = mg.ToGuid();