Open Windows Firewall, click on "Advanced Settings", look in the list for an entry named "File and Printer Sharing (Echo Request - ICMPv4-In)" and enable it. Your machine should now respond correctly to IPv4 pings. There is also an entry for ICMPv6 as well if you are using IPv6 on your network.
If you are using a firewall program other than the built in Windows Firewall, let us know what firewall program you are running and we may be able to assist you in allowing ICMP packets through it. :)
IT Desktop & Network Consultant - MOS Master Certified, MCP Certified, MCTS - Windows 7, CCNA Certificate Pending, A+ Certified.