Poor CPU performance and High RAM usage due to unknown background applications

My computer is hanging again and again. You can imagine the severity of this problem as I have to use Alt+Ctrl+Delete at least 6 times in an hour. There are various softwares running in the background which are not known to me. I opened up the task manager and saw the memory usage. There are several apps which are taking 30% of the RAM usage and I don't know even a single app from those. Please tell me some of the unwanted background applications, which can be closed to increase the CPU performance.
