When you buy a new PC it is loaded down with un-necessary commercial software such as anti virus packages. This is because the PC manufacturer has been paid to put them there and is probably making more profit from them than from building the PC. They are best removed.
As you use a PC all the software you use, including your operating system, web browser and search engine, writes stuff to all sorts of different places on your system, without asking you. Commercial websites and online marketeers put their own stuff on your machine, mainly to spy on you. All this rubbish slows your machine down till eventually it becomes like stirring porridge. You need to take action to fix this.
Utterly essential is to install CCleaner (all you need is the free version) and to update it regularly. Run the Windows, Applications and Registry cleaners regularly and you will be shocked how much rubbish they clean out. And you will be very pleased at how much better your computer runs.
You need anti virus software, there are a lot of malicious and stupid people out there. Do not spend a penny of your money on this. If your computer has paid for anti virus software remove it. The people who understand PCs best are Microsoft and their fantastic anti virus package, Security Essentials, is totally free. Set it to give you real time protection, to update daily and to do a full scan of your system daily. Now you are fairly bullet proof.
Not many people know this but Microsoft have an even more powerful tool to check and clean a PC, called Safety Scanner, which gives your machine a massive health check. Very useful on a machine that has been used for a while.
Windows is also fitted with a firewall to protect your machine from attack. You should know where this is and what the settings are. This is a basic necessity of life yet many are unaware of it.
After this you still aren't totally safe!! There are two more packages that are essential and complimentary to one another.
Malwarebytes Anti Malware removes all sorts of malicious software from your machine. Always update it before using it. Check your computer about once a month. You will be shocked at how much stuff has been put on your machine by third parties without your permission. Once again the free version is all you need.
Spybot Search and Destroy is fantastic. Run it after all the other stuff and it still finds loads that you don't want on your machine. Once again update it before monthly use and only use the free version.
I have real world experience of using Security Essentials, Malwarebytes and Spybot Search and Destroy to get rid of very nasty infections and they did it. Also millions of computers are zombies, used by criminals without their owners realising it. It is inevitable that many people reading this will have zombie machines. Install and apply all the above and most zombies will be removed.
If your computer has been in use a while a good way to speed it up is to defragment the hard drive. Go start menu > all programs > accessories > system tools > disk defragmenter. This reorganises all your data in a more coherent form so it loads more quickly.
Next we come to the software you use every day. Much of this is very insecure and can be easily hacked. By government security agencies and by malicious criminals and recreational hackers. White Hat Aviator is designed purely for maximum security and is the perfect browser when you are dealing with anything confidential, like online banking or shopping.
Disconnect Search also is a lot more secure and Aviator uses it by default. Look at the settings so you know how to delete stuff like browsing history. With this browser and search engine together your web presence become near to invisible and only the minimum record will exist of your passing.
Finally beware your webcam. For a long time NSA and GCHQ have been able to watch you any time they want, even if the software is turned off. They do the same with the cameras in your phone and tablets. Now hackers can do the same. Your webcam is a window on your home for the whole world to look at you through, without you ever being aware. So point it at the ceiling or cover it with insulating tape!
Everything above is very easy to do, even for the non computer expert. There is more if you want to be more anonymous and have more computer skills. Tor, for instance and Proxy Servers. But this really is an arms race against NSA and GCHQ, who have more resources. Against malicous and hobbyist hackers it is far easier to stay ahead.
Finally all the "free" software above contains huge amounts of human labour. They all have donate buttons and they are all excellent causes, it does a lot of good if you occasionally contribute to the people who work so hard for you.