That limitation seems to only apply to their free product.

Quote Originally Posted by ZoneAlarm website
System Requirements for all other ZoneAlarm Products*:
Microsoft® Vista SP1:

* 2 GHz 32-bit or 64-bit processor
* Minimum system RAM: 2 GB (32-bit) and 4 GB (64-bit)
* 250MB of available hard disk space. Internet access

* Free ZoneAlarm Firewall is not available for Vista 64-bit
Microsoft® Windows® XP SP2, SP3, Home or Professional Edition:

* 1 GHz 32-bit processor or higher
* Recommended system RAM: 768 MB
* 250MB of available hard disk space

ZoneAlarm security software works with most types of TCP/IP connections including Ethernet LAN, wireless LAN, DSL, cable modem, and dial-up connections.

* Supported Protocols for Email Protection
* Support for Parental Controls
* Operating Systems Support
* FAQ for ZoneAlarm End-of-Life Policy
So yeah, if you somehow find a "solution" to it, it might work.