Windows 7 Professional, Ultimate, or Enterprise has the option to use Virtual XP mode. This article discusses OfficeScan client's compatibility with this Virtual XP mode.
OfficeScan 10 Service Pack (SP) 1, 10.5, and 10.6 are compatible with Windows 7 XP mode.
The host Windows 7 and the guest XP are treated as two independent Operating Systems. To protect both of them, you need to install an OfficeScan client on each of them. These clients will count as two seats.