Well, with a VM, even with an encrypted virtual disk, you're still leaving evidence on your hard drive. VirtualBox will leave logs of when the VM was run, which can be used to correlate your activity. Of course, someone could look at your Windows logs of when you boot up and shut down and infer that for a bootable OS too. Really the best option is a dedicated computer for sensitive stuff, but again that's not practical for a lot of people.