Anyway, I decided to try one of those little tiny free heap-trackers, specifically Fluid Studios Memory Manager. It's just a little #include and piece of C-code that logs allocations and reports on any bonehead stuff you've done. It works beautifully, finding a memory leak in my stuff almost immediately. In addition, it's got a couple of #define's to do extra stuff like logging all allocations or even randomly failing so you can see how gracefully your app handles a crash. It's 100% free, so you've got nothing to lose in using it.
Note, though, that this only tracks new() and malloc() for you. The commercial tools, like the aforementioned Bounds Checker, find lots more stuff for you, like un-freed GDI handles and such. Still, you can't beat the price and I bet it'll find something in your code.