VirtualBox
yeah this
Sometimes this depends on the OS you're looking to emulate.
Windows,
windows what?as an example, windows 98 would not work extremely well on virtualbox unless you knew what you were doing, but windows 8 would work fine.
Actually, the opposite applies. In my experience, Windows 8 emulates horribly with VirtualBox (especially with hardware-accelerated graphics), but I would expect 98 to work mostly fine (although I haven't tried it lately). However, as I said before, VirtalPC or Hyper-V might be a better option for Windows-on-Windows.