With the way you are currently setup, what you wan't really can't be done.
I'm assuming that you ar using the DHCP function in Untangle vs. on some other device (DSL modem, router, etc.).
If DHCP is coming from some other device, then the answer is it definitely can't be done (unless you want to get cozy with the coders that work for whatever vendor created that device.)
If Untangle is providing DHCP, much depends on how DHCP was implemented. All DHCP really does is assign an IP address to a particular MAC address. It may or may ot capture a computer name for that MAC address.
DNS (if you have it configured in untangle), is the only thing that might tie an IP address to a specific computer name. However, in order to capture IP addresses provided by DHCP, DNS would have to allow dynamic updates. Not all DNS implementations allow for this.
Even if all of the above is working in your favor, neither DHCP, nor DNS will capture what user is logged into a computer with a particular IP address. The best you could expect would be to tie a particular activity to a computer with a particular IP address and name.
You would have to then search the event logs of that particular computer to deterine who was logged onto that computer at the time in question. And that assumes that you have Security logging turned on (by default, it is turned off in XP). This of course also assumes that you aren't using any universal logons, and that users don't know each others credentials.
Untangle does have a User Access and User Authentication capability, but this is only used in relation to Remote Access capabilities of Untangle.
The one capability of Untangle you need (Active Directory Authentication with Reporting and Policy Enforcement), you are denied because of your current network environment. This capability would allow you to generate a report showing user activity based on user ID.
To take advantage of this capability you would need a machine running a current Windows Server OS set up for an Active Directory domain environment (which may or may not make your Untangle server superfulous as the Windows Server could perform most functions of the Untangle server).