After Hours of searching I finally found the answer to releasing the EXCEL.EXE process from task manager.

If anybody else has the same problem, the code I used is :

oWB = Nothing
oSheet = Nothing
oXL.Quit()
While (System.Runtime.InteropServices.Marshal.ReleaseComObject(oXL) <> 0) ' Repeat until no more COM references
End While
oXL = Nothing

' Invoke garbage collector before termination
GC.Collect()
GC.WaitForPendingFinalizers()

Thanks

Steve