I'm running Zabbix Agents on several Windows Servers (64bit, 2008+) and they all overflow at some point when querying net.if.in[nic] and net.if.out[nic]:

Fri May 8 12:31:56 EEST 2015 2005196881
Fri May 8 12:32:02 EEST 2015 2527914095
Fri May 8 12:32:07 EEST 2015 3053020449...