VM software that can boot to Windows partition created by Boot Camp
Both VMWare Fusion and Parallels Desktop support running VMs directly from Bootcamp partitions. They also offer migration from a partition to a VM image if you ever want to go that route.

It appears that it's also possible with Virtualbox, but not a supported feature. I found this blog post detailing how to configure it (the post also lists several other sources that may be helpful).

Relevant support articles:


Launching your Boot Camp partition in VMware Fusion
Parallels: Using Boot Camp Partition in Virtual Machine