As i see there could be two possiblities here .
1.You are not completely shutting weblogic server down.
Try
http://localhost:7001/console in browser and see if you are getting username/password dialogue.
If you get this dialogue box the server is still up.
2.Secondly you said you have changed config.xml
Did you by any chance copy this from other weblogic instance elsewhere?
If you did so then also there is a possiblity that it may give errors for security reasons.
Please provide the following in your next post.
1.error you see in the weblogic
2.Changes you made between the first version of xml and second version of config.xml