If you want to emulate Win95/98/Me, use qemu, vmware player* or vmware server. Virtualbox does not support Win9x series. I would recommend vmware player as it's easier to use than the other two emulators.
For emulating Win2000/XP or later, any of the emulators you can find should work. Vmware products now have experimental Direct3D support, and Virtualbox has seamless windows mode.
*Use easyvmx to create new disk image and vmx file.
http://www.easyvmx.com