Windows 2003 Enterprise Server was not responding to PINGs

As a network administrator in my company, today I faced a problem which made
me restart Windows 2003 Enterprise Server (no SP but up-to-date hotfixes).
I was pinging it from some clients but it was not responding the them and
seemed to be disconnected from the network. Everything recovered when I
restarted the server.

And something amazing, meanwhile my own workstation (Windows XP - SP2 with
up-to-date hotfixes) was unable to ping others! I found that IPv6 is
installed and my client was able to PING after to uninstalled IPv6.

Actually I don't know what had happened but the two problems - Windows 2003
Enterprise Server and Windows XP client - seemed to occur in the same time.

Any comments or helps would be greatly appreciated.
Jamal