Security sometimes is about to clean the OS from all the things that may constitute a threat for the system and limit the connections between programs and user capabilities. This is fundamentally why my Windows Phone is theoretically more secure than my Windows 7 installation: because WP, by default, is (sometimes insanely 😀 ) more sandboxed and limited respect to the desktop OS.
But if the user needs to have a certain software distribution to enjoy his computer is also true that the user executes tasks that needs effective security. E-commerce, banking, Tax and Government procedures, personal documents editing and so on, simply can’t be done on the same system we use for gaming, chatting, explore the Internet and maybe using pirate softwares.
We need a Secure Box, a strong system used only for matters that needs more security than normal. No multimedia if not needed, no daemons or services if not needed, system completely locked and hard to modify, obviously fully encrypted to protect the important data and with more security checks.
The reason of have two systems is clear: you have to be sure that the hardened system remains secure and difficult to attack; this, from a desktop user perspective, is impossible to achieve on an OS full of vulnerable, not enough tested, software and where user operates according to vulnerable behaviors. Just switch OS when you need to do some activity that needs more security and a clean system.
As for example you may use a GNU/Linux Ubuntu for daily activities and then switch to a minimal GNU/Linux Debian stable of CentOS installation for security/private/financial tasks.
If you don’t care about maintaining certain data in a more enforced OS, then you should think to use a secure GNU/Linux Live CD to operate on the network where security of the system is crucial.
For example you may use TAILS that is centered on anonymous surfing or LPS (If you trust US Department of Defense) that is centered on system security but you can also use a more friendly Knoppix too! In this case, you know that even if a session is corrupted by malware or hacking attempts, you can reboot to have a new clean system to use.
That’s up to you! I don’t advice you to use the same home system you play and websurf with, also for crucial security tasks.