Sometimes a memory related problem will cause behavior such as that.  I am talking about a physical issue with the memory in the machine.  The fact that the end result varies tends to say it's not a software issue.  Carrara uses a LOT of memory, just like any 3D modeling app.  I was having a similar issue with C6 Pro when I built the machine I am on now.   It trurned out to be 1 bad DIMM.  I replaced it, and no more issues.  I have been running C7 Pro for a few months now on this new system and just upgraded to Windows 7.  I find that it is actually more stable under Win7.  I just wish they had created a 64Bit version of C7 Pro.  It's not like 64Bit computing is that rare anymore.  Get a memory diagnostic program and do a comprehensive memory test.  It may be an upper memory location is bad.  Most apps don't use as much memory as C7 so they might not trigger the issue.  If you could post some more details about what hardware you have in your system, I could try and come up with some more ideas.

Good luck.

Rich