I have a broadband connection with a WiFi router. I have installed a new PC at home and it is working well and is able to use the internet. The problem I am facing is to share the data from the old one. The new OS is Windows 7 and the old one is Windows XP. I just want to transfer all my data from the Windows XP machine to the Windows 7 one, so how can i use the WiFi network for this purpose.
If you want all your personal data (My Documents, etc.) on the new machine, then you can enable file sharing on the old XP computer and copy/paste them from the new computer. A description of the steps to take to do this can be found here (Microsoft Support).
Another option is to map the relevant folders as a network drive, then copy/paste the files out of the network drive to the new computer. The How-To Geek has an excellent article on how to map a network drive between a Windows 7 and a Windows XP machine.
Mapping a network drive not only gets the information to your new computer, it creates a permanent link between the computers to share the info. Just realize that the network drive is like making a bridge - it doesn't move the data, just let's you access it. But once you access it, you can move it anywhere you like.
I prefer to use filezilla server to ftp locally within my home network.
A free dropbox account is also commonly used to share between computers.
I have found many problems / bugs with trying to use window's networks across different O/S' - not to mention the potential security issues.