Shazel:
Well, as Raptor stated, your problem is with the Point-to-Point Protocol (PPP), which is a way to connect your computer to your Internet Service Provider (ISP) so you may access the Internet. If you don't have the correct configuration settings provided by your ISP, you will not be able to connect to the Internet through your ISP. These settings often include a user name and password and other security settings.
When you reinstalled Windows XP, these settings were likely changed. Two ways to change them back:
1. Call your ISP and have them talk you through making the correct changes yourself.
2. Use the installation CD provided by your ISP to reinstall your their software and PPP configuration.
Regards,
Doc