There are several advantages and disadvantages of having a real time operating system. One disadvantage is unseen errors, an advantage is timing.
A single user OS could also be a real-time system. The differences in the two are based on how a process is given control or priority within the system. Realtime systems give preferences to realtime control processes (realtime runs at a much higher priority level). Most single-user operating systems treat processes with equal priority unless you indicate that a certain process is to receive preferential treatment. This can be done by raising the priority of the user process to that of realtime, meaning it will receive resources (such as CPU cycles) above other programs in the system.
No. Windows is (regrettably). Mac and Linux are the other main contenders.
Some versions of Unix are oriented towards real time applications, and processes in Unix can be "promoted" to real time status if desired. Other than that, you would have to define more precisely what you mean by real time for an operating system.
Realtime Worlds was created in 2002.
Realtime Worlds ended in 2010.
Realtime Worlds's population is 200.
Realtime Interrupt was created in 1995.
Realtime Associates was created in 1986.
Marooned in Realtime has 270 pages.
Marooned in Realtime was created in 1986-09.
XP is a complete operating system, not simply an operating environment.