There are numerous free and commercial physical-to-virtual (P2V) conversion tools on the market, but often they require some setup or at least a minimal initial investment.  Sometimes, all you want to do is make a copy of a physical disk and then attach it to a virtual machine.  The Disk2Vhd utility is a simple program for doing just that.  Here’s the info from the TechNet site:

Disk2vhd is a utility that creates VHD (Virtual Hard Disk – Microsoft’s Virtual Machine disk format) versions of physical disks for use in Microsoft Virtual PC or Microsoft Hyper-V virtual machines (VMs). The difference between Disk2vhd and other physical-to-virtual tools is that you can run Disk2vhd on a system that’s online. Disk2vhd uses Windows’ Volume Snapshot capability, introduced in Windows XP, to create consistent point-in-time snapshots of the volumes you want to include in a conversion. You can even have Disk2vhd create the VHDs on local volumes, even ones being converted (though performance is better when the VHD is on a disk different than ones being converted).

You can download Disk2Vhd directly from the Microsoft TechNet SysInternals site (it’s only 704KB).  You can then run it directly from your file system to launch the GUI.

While it’s easy to create a VHD, it’s important to keep in mind potential issues with using this approach to “clone” an already-running machine.  Depending on your environment, you might need to consider the impacts of network addresses, MAC addresses, computer names and Security Identifiers (SIDs).  Most of this only applies if you’re using this approach to clone a non-OS hard disk.  Overall, the utility makes it really simple to perform a P2V conversion of a hard disk.