It is mainly to do with Services that run at any one time. SuperFetch for example slows things down significantly (well in my language it does, in normal use probably not!)
Try command prompt, and then type SC QUERY. That will show you all the services running on the computer. Then you can go into services.msc (type services.msc) and manually adjust which services load on startup.
If you want to go even more technical, you can create 2 batch files (.BAT) called start and stop and one loads everything you regard as normal, and the other stops every possible service (for ultra criticial speed). Using SC START "Service Name", SC STOP "Service Name".
However if you are just trying to unclog your computer, I would try to get to know what services are running and disable any which are not needed. You can also access the Start-Up tab on the Task Manager, by pressing Ctrl+Shift+Esc. From here you can disable certain programs or utilities running on start up.
Since I work with music and video editing, I need my computer to be 'waste of resources' free.
I don't bother with external antivirus programs, Windows Defender is the most efficient and it's part of the operating system (not sure if you get on Home edition though?)
As for MalWare, I might once in a while use the free SpyBot Search&Destroy to double check my PC, I'm not really taken in by the need for live malware protection but that's just me. Just never click on dodgy links or popups, that is what I stick to. If you use a good browser (most of them are good these days really) they already tend to have warnings in place for bad links/pages online.
