MTBF of complex systems is theoretical easy to calculate. Yo should divide the system in small pieces and divide in series system and parallel system.
In serial system, if a component fail, all system will fail. In parallel system, the system maybe continue runinf even if a component fail. ie, your floppy disk drive is broken but the computer still working, execpt you can not read and write floppys.
I describe how to use Excel to calculate the MTBF of a product in my book
"Excel by Example : A Microsoft Excel Cookbook for Electronics Engineers" published by Newnes/Elsevier, ISBN 0750677562.