Press Control, Alt and Delete all at the same time.
This will show the task manager.
Applications tab shows the applications running.
Processes tab shows what is running.
You can click the names of the columns to sort and see what is using the most resources (either processor or memory).
Some of the names might look encrypted to you. Before you start turning things off make sure you know what they are. Some are part of Windows.
Some processes could be hogs too. So investigate names you can't figure out by looking at them to see if you really need them running from startup.
You can change the schedule of things like your anti virus scan. Usually by their interface but if you can't find where to change it in there, you can look at the
Task Scheduler and make changes there.