Most people among us change their IP address and thinks they are anonymous on internet but one thing they forgot that they can be tracked down with MAC address which is still there non spoofed. So what to do then? Its practically impossible to change your MAC address but yes virtually its possible :). Here is a application called SMAC that can help you to spoof your MAC address to maintain your anonymity on internet.
SMAC is a powerful, yet an easy-to-use and intuitive Windows MAC Address Modifying Utility (MAC Address spoofing) which allows users to change MAC address for almost any Network Interface Cards (NIC) on the Windows VISTA, XP, 2003, and 2000 systems, regardless of whether the manufacturers allow this option or not.
SMAC does not change the hardware burned-in MAC addresses. SMAC changes the “software based” MAC addresses, and the new MAC addresses you change will sustain from reboots.SMAC helps people to protect their privacy by hiding their real MAC Addresses in the widely available wifi Wireless Network. SMAC also helps Network and IT Security professionals to troubleshoot network problems, test Intrusion Detection / Prevention Systems (IDS/IPS,) test Incident Response plans, build high-availability solutions, recover (MAC Address based) software licenses, and etc
Download SMAC here