Well it is possible to run a virtual pc on your computer which would have its own partition and information on your HDD but you would need a fairly powerful computer to run it at a good speed. It is also possible to install multiple OS on your computer at the same time and when it boots up you can choose which OS you want to use, they would all have to be on different partitions and sometimes you also need to install them in a certain order as well otherwise the MBR(I think thats what it is??) will get all screwy and you will most likely have to redo the whole thing. The most OS I have had installed at once on my PC was 4 and I'm not sure what the limit is but I would say that 4 is more than enough anyways.
Hope this was helpful, if not maybe I can try and explain it a little better for you, just let me know.:iagree: