Questioning my Computer's Security. Need Suggestions.

[Pokeventurer]

Hi guys today i am writting this up to give you guys general info on my com's situation and what security measures should i take for it. so i have been using a Dell Laptop (idk specifically which) and i am super anxious about every tiny bit of details that are going wrong with it, and yes i am just ultra conscious about it's safety; i have not downloaded any anti-virus programs yet because either it turns out to be a virus or it slows down my PC and gives me more probs than solved (past experiences). all those weird things i downloaded led my laptop crippled as it won't shut down and when i sent it for repair the guy that my mum knew and had contact with for such stuff took out the original battery and putted in the one that runs like half the original, he tells my mother that it was a problem in the charging of my laptop, when actually i had noticed malfunctioning in the past two days ( this brute was caused by my sister who just to annoy me did something to my laptop the morning she left for college when i was asleep ) and over that stuff my mom won't believe me. i don't wanna end up feeling bad for myself and being frustrated or angry; therefore i felt like taking a step to properly get the job done.
So now again i have been seeing a iny-tiny bit of weirdness that doesn't do something bad to my system or it's functioning but it seems kinda somewhat infected. i have heard alot on MSE: Microsoft Security Essential being an excellent and free software designed for malware i have also heard that it is lite on your system and doesn't effects it's speed or regular functioning * features i want in one* so can i please get a bit more information on this program and where can i download it for windows 8.0 and keep my system safe; help will be appreciated alot!
On another side relating my computer a message pops up if i wanna update to windows 8.1 for free and it is available in my store; what is this message abut, it'll e good for me to know a bit more about it and what are the benifits of taking this free update oppurtunity and all the related pros and cons.
if you are wondering about my pokemon competitive side i am writting up a analyses which will soon be up, and i am currently feeling weak on teambuilding a strong team, but i'll soon be back with one! Bye for now!