You can have multiple OSes on one computer. I have XP, 32 bit Windows 7 Home Premium, 64 bit Windows 7 Home Premium, 32 bit Windows 7 Professional, and Windows 8 Pro all on one computer with multiple boot. You can make a DVD or flash drive with Ubuntu, and you can make a bootable XP CD with UBCD (Universal Boot CD). Windows 8 WILL make a repair volume on a flash drive, but it's nowhere near a full Windows 8 OS. If you want to get into virtualization, you can do all kinds of things, but the ones I'm familiar with (XP Mode, Hyper-V, and Oracle Virtual Box) all require having files on the hard drive. Since hard drive space is so cheap these days, there's no reason to have to fool around with putting things on a flash drive except for portability.
There may be others I haven't gotten into, but the above gives you LOTS of flexibility.
I own an Acer laptop that is runing Windows 7. I love Windows 7 and finally made the move from Windows XP but I really like Windows 8. Yet I'm nowhere near ready to make it my only OS. I know one possible option is a dual boot system. If I need to go that route I may be posting again in the future to find out how I can make a dual boot Windows 7/Windows 8 system. Yet I like trying different OS's and already have two USB sticks that are ChromeOS and Ubuntu Linux. I purchase 4GB flash drives and buy the fast drives.
The question is is it possible to install and then boot from a Windows 8 USB Flash Drive? If so, how would I go about doing this? Also, what is the minimum size USB Flash Drive that could be used? I want at least some room to work with and install apps but it won't be my primary OS. If it's not possible to do this how would I go about making a dual boot system? Already I have a 500GB HD with Windows 7 installed but I believe I'd have to repartition and hope to avoid having to reinstall Windows 7 since I have so many installed apps.
Thanks for help with the above.