How to Switch Between Operating Systems Without Reboot?

Not as I think you mean. The only way to go from a proper "bare metal" install of Ubuntu to a "bare metal" install of Windows is through BIOS.

However, virtualisation might be an option (it is for me). I only have a few Windows applications that I occasionally need for work (Photoshop, Illustrator, Fireworks) that don't work acceptably with with Wine. I use VirtualBox to boot a "virtual" install of Windows. Note this requires quite a powerful machine with plenty of RAM to run really well.

Wine is another option. Wine allows you to run some Windows applications on Ubuntu but the coverage is a lot lower for most applications than a virtualised environment.

If you explain what you're trying to do in Windows, perhaps we can explain the best of those solutions.

Using a "Bare Metal" hypervisor for virtual machines (such as VMware) may allow you to do this. There is a cost in resources of course. This is not the sort of VM where one OS is the host and another a guest ... both are equal, and run under a thin low-level hypervisor.

Run one operating system as a guest in a virtual machine, with the other system as a host. (You can also run both as guests inside a hypervisor.)

If you want good integration between Windows and Linux, and don't mind running Windows as the primary operating system, you can run coLinux, which is a virtualized Linux running on top of Windows. There is an Ubuntu-based distribution of coLinux: andLinux.

No, not with a dual boot setup. The only way to come close to this is to install Windows in a virtual machine using software such as Virtualbox. Virtualbox can be installed from the Ubuntu Software Centre (just search 'virtualbox').