You can have both drives in simultaneously, just use the BIOS boot menu to select the proper drive to boot to if you need. When booted, you can open Windows' Disc Management program and remove all of the partitions on the drive you want freed (i.e. the drive you did not just boot in to). Be aware, though, a stark change in hardware may cause an old Windows installation to fail to boot, so moving a system drive from one old PC to a new PC may not (and likely will not) just work. As always, if you are creating a new system, then clean install the operating system to avoid any issues.