The more complex the system, the harder it is to secure. Apple’s Mac OSX has had a capability called FileVault for a while now. The basic idea is that it encrypts your home directory and everything that lives in it, including mail, browser cache, private application state, and so on. It’d be the windows equivalent of encrypting HKEY_CURRENT_USER plus all the files in your user’s directory.
A hint posted recently to Mac OSX Hints shows a problem with FileValut and Mail that leads to an information disclosure in a very public place on the box (/tmp). You can argue about how bad this really is, but I’m more interested in the pattern here. It’s not enough that information be hidden in its final resting place; it has to stay hidden at all points along the way. This is why people worry about encrypting page files and protecting memory (which was part of the Orange Book C2 standard that Windows qualified for back in the NT4 days).