Just also wanted address

The same program runs flawlessly on 2007 iMacs running 32-bit Windows XP through bootcamp
If the same experiment runs perfectly on one system but causes an overflow on another--using SAME version of DirectRT, then the FIRST thing to look at is the video card. Depending on the year and model, some Macs come with nVidia cards, and others with ATI Radeon HD Cards. Take a look and check the video cards on the older XP vs newer Windows 7 machines. Again, an upgrade would fix it if this is the issue, or a video card swap would also likely fix it too. In fact we had this SAME issue here last year when we couldn't figure out why v2010 ran fine on one of our iMac but not on our MacBook Pro--both running Windows 7 on Bootcamp. Turned out that the MacBook Pro had an ATI Radeon Card and the iMac had an nVidia card. That was it.