If i'm not mistaken, DLLs always run in an EXEs process space. If you can get another app to call a Function (c/c++) or a Method (VB) in your DLL, then it will not be shown. The trick is to get the EXE to call a Function/Method in your DLL.

Try making it a plugin of something. Winamp and Photoshop plugins run in the process space of Winamp and Photoshop, respectivly.

To my knowledge, making it a DLL is the only way to make it a "child process" of another process.