Windows is not a real-time operating system. Real-time operating systems have timer mechanisms that allow the system to make hard guarantees about when timer-initiated events occur and the overhead associated with them, and allow you to specify what behavior should occur when the deadline is missed -- for example if the previous execution took longer than the interval.
I would characterize the Windows timers as "best effort" when it comes to smaller intervals. When the interval is sufficiently long you don't notice that you aren't getting the exact interval that you requested. As you get closer and closer to the resolution of the timer (the frequency at which the timer runs), you start seeing the overhead as a percentage of the interval increase. Real-time systems take special care to minimize the software overhead, relying on more sophisticated and faster hardware solutions. The exact frequency of the Windows timer depends on the timing services that the underlying hardware provides and so may differ from system to system.
If you have real-time needs -- and doing something every 50ms may fall into that category -- then you may need to look at specialized hardware and/or a real-time OS.
