What are the advantages / disadvantages of using cooperative linux like portable ubuntu for example compared to a qemu or any other virtual machine installation. Is one option notably faster than the other plus and other things that should be taken into consideration.
I found both options to be rather slow. The problem is that both run all the code associated with the guest on a single thread, even if you have a multi-core machine and for the host OS it looks like "any other thread" which can be preempted by other threads on the host OS...
I'm currently using a Colinux-based Arch Linux system. I like it because it's very lightweight compared to a standalone edition, especially of system such as Ubuntu.
Colinux is currently single-core only, newest VirtualBox supports multicore (if your CPU has VT-x).
Graphics can be tricky. If you use Colinux with simple X11 forwarding (connecting it to a Windows X-server like Xming - I believe this is what andLinux and Portable Ubuntu do), you get a nice 'seamless mode' - Linux applications run alongside Windows ones, not in separate window; you don't need a separate window manager - but it isn't as responsive as VM would be.
You can also set up a VNC server in the guest and log in from the host. No seamless mode, but it's more responsive and can be overall more comfortable. Still no chance of 3D acceleration, though.
Due to not having to run X/window manager/various daemons, Colinux is noticeably lighter on RAM. I believe it's also faster, but I haven't tested much.
I've succeeded in running the same Arch installation (from a separate partition) both in Colinux and natively - look here for details on such installation. However, I think VirtualBox can do that too.
Be aware that you can't use both at the same time - Colinux refuses to run alongside a VM.