JCO Error

I am able to connect to R/3 Enterprise version using my JCO program with JCO version as 2.1.5. But I am facing a problem when I am trying to connect to R/3 4.6c. I get the following error:

com.sap.mw.jco.JCO$Exception: (103) RFC_ERROR_LOGON_FAILURE: Name or password is incorrect. Please re-enter

at com.sap.mw.jco.rfc.MiddlewareRFC$Client.nativeConnect(Native Method)

at com.sap.mw.jco.rfc.MiddlewareRFC$Client.connect(MiddlewareRFC.java:1098)

at com.sap.mw.jco.JCO$Client.connect

But I am able to login into SAP GUI with the same userid and password.