The Microsoft OS OEM license allows you to install (and in some cases, virtualize) the software only on the originally licensed machine.
Even the new OEM license (the one on new machine on which you wish to run the VM) won't solve your problem. While you probably have Professional license (or higher) granting you downgrade rights, you won't actually have any virtualization rights (these are applied only to volume licenses covered by Software Assurance and some other minor cases, read more).
As to the technical issues, you may easily convert old hard drive into virtual machine drive. If you plan to use VirtualBox, you may do it easily with Disk2vhd app.
You have to remember though, that if you convert single partitions, you will need to rebuild MBR using recovery tool on the installation CD. Please review the following article for further advice on this topic: How to create vdi from existing (off line) partition?