Tweaking is a term that refers to optimizing the performance and features of a system, in this case the computer.
This can be achieved by modifying an application's source (but programming skills are required) or by using a dedicated application.
There are specialized apps that deal exclusively with the PC startup, interface or logon speed.
There are some software solutions that can boost the overall performance of the system or personalize Windows completely.
It should be mentioned that it is advisable to create a restore point before applying any tweaks, just in case something goes wrong.