Multi task is ability to maintain schedule for more than one process. As normally number of processes exceeds by far number of CPUs in a system, the system scheduler becomes the core part of such multitasking. There is no real multitasking though, as only one process occupies processor's registers at time. However as it happens relatively fast it creates illusion of many processes flowing simultaneously. Multitasking systems is an opposition to mono tasking system as MS-DOS was. While in MS-DOS it was possible to have some processes to simulate simultaneous execution it was dome by using range of interruptions that is not a part of the system and worked more as a switch, while one process was working, until user would not designate manually recourses to the next process it would keep the current state.
Yes Windows XP is by all means a multitasking system.
Multi-User system implies that system can handle simultaneously more than one user. It means that system knows how to recognize the user, how to keep the environment and how to separate recourses between users.
Yes Windows XP is by all means a multi-user system.
The both Windows XP and UNIX may have many users connected using different hardware means and having sessions simultaneously.
I suspect you ask it as a point of comparison to Window/UNIX advantages/disadvantages. Let me tell you that the major advantage of any UNIX system is its simplicity. UNIX has very simple architecture compounded of many subsystems. If something is going wrong you always can trace problem to the specific subsystem. Each subsystem is mostly independed and may be stopped/started/ refreshed with no influence to anything else. Each subsystem is open, it means it may be discovered by regular mortals simply by reading man page; it has no "magic" involved. UNIX despite its evolution had some revolutionary steps in development and does not have to take increasing care of backward compatibility in its worst sense, because most of the programs can be recompiled with little efforts.
As system is modular it is easily upgradeable, many different teams of developers can coexist with little disturbance to one another, they don't have create “the system”, they are responsible for a little part and this way makes the system in general very reliable. There are many competing projects; each try to prove its module is the best.
Sorry for tedious port.