1. Do we need to set one ODBC conneciton per company or there is any way to change the company of ODBC Connection using programming (I am more interested in C#.NET)
The company file is selected by using the DFQ = Path to company file or . (dot) to indicate currently open company file parameter in the connection string see: How do I build a connection string with all the available QODBC parameters? for more.
2. Can we call QODBC from the windows service? I think yes. but my problem is when the service is accessing the company file, is it possible to open QB UI for the same company ?
Yes, and the QuickBooks UI can be set visable if required by checking the "Launch QuickBooks UI" option.
3. When QB is open and QODBC is accessing the company file, we cannot close QB until QODBC finished accessing of the file. is there any workaround for that ? or we just have to wait for QODBC to finish its operation ?
No, you must close the connection and in the case of Windows Services, wait for the Windows O/S to really close the connection. BTW: QuickBooks can only have one company file open at the one time.
4. Can QODBC work on the machine on which QB or QB SDK is not installed.
QODBC talks directly to QuickBooks and not the companyfile.qbw file, so it needs to find a copy of QuickBooks. | 
