Hello Ashok9,
Explanation to a part of your problems, is that, when you connected your system, Windows XP, as you said, to internet, it automatically downloaded an update from Microsoft, which discovered, that the operating system, you are using, is illegal, so in that case it continued to drop these notifications for you. It can be removed, but it's also illegal, so, first you will need to buy a Windows Genuine Kit for validating your Windows copy.
About all other problems, i'm sorry, but you will need to be more specific. For example, what software are you using for getting Internet Connection from your USB modem?
Edgars