I need to know the disadvantages of an operating system
That would depend on what type of operating system you mean.
Windows: Not many disadvantages at all except that over time the operating system can degrade if not looked after properly. For example bad drivers, corrupt boot files etc.
There are tools like defragmenter and other registry tools that can keep the operating system in good working order.
Linux: This can be a very difficult operating system to work with as unlike Windows you have to know "Source Code" this is the input languge that is used with OS's like Linux, Unix etc.
Mac OSX/leopard: Very similar to Linux but the difference is that Mac is mainly just a tool. Used mainly for people on the go and who have no time to work properly with the operating system. Mac has specialized applications which are used all around the world. Many government institutions like the Police use Macs. Mac's are also the most expensive of all the operating systems and computers.