I reciprocate Jack's agreement. My laptop is 5 years old; paid $1100 for it in 2009. I'm a hardware engineer. It still runs faster than most people's brand new laptops. Hell, I've seen people buy a better laptop than mine, after I bought mine, and replace it, all within the time I've had mine.
Why does mine run fast? Why is a hardware engineer ok with a 5-year-old laptop? I mean, I'm not just office-and-internetting on it, it's a full-time dev machine - software, hardware, firmware, batch image processing, CAD, whatever.
- OS. As Jack said, a clean wipe and install of an OS does wonders. I partition my hard drive and once every couple years I get a new OS, slowly transfer data off the old one, and eventually shrink it to about 30 GB (under 5% of the disk size) and leave it frozen in case I ever need anything from it.
- OS. Okay, so I'm cheating: I run linux. I upgraded from one major release to the next and it actually ran faster. When did windows ever run faster on the same hardware after an upgrade? Right. Also when I upgraded to a new cut-down distro that was a derivative of the old one I used, I went down to a 10-second boot time... with a traditional hard drive.
- I clean it. Every couple months I open my laptop and blow all the dust out.
- I upgraded the ram once. 4GB to 8GB.
- I got a new HD once. First one was dying since it was brand new; it got bad right during the time the HD factories got flooded so I got fucked. Still, a new hard drive helped too, though it's not really necessary...
- I get rid of the cruft on my operating systems often. Yeah, I have windows installed; it runs fast because I don't let it get bogged down in bullshit.
So I can take just about any old PC and vacuum the shit out of it, maybe see if it needs a new fan (usually not but my laptop had bad luck with fans), do a clean wipe and OS install, and keep malwarebytes and windows defender / whatever antivirus you use regularly active.
One more thing. Get a $10 flash drive. Get a free program called Unetbootin. Use it to install an Ubuntu live image to your flash drive; this is free; ubuntu is another OS. Now turn off the computer, plug in the flash drive, boot it up, and you get something that sort of looks like a cross between windows 7 and a mac.
Use that to watch porn.
Seriously - porn is probably the #1 reason a computer slows down. Now, I know, I'm not implying your computer is being used to watch porn, I'm not implying ownership of the porn, I'm just saying, if someone watches it, then use the live image. All the malware and viruses and trojans that try to run - well, #1, they can't because they're almost all written for windows; #2, the live image erases any changes you make every time you reboot so it's always clean, #3 it would be an insanely impressive virus that will run on your live image, skirt under the pretty damn good security of linux, recognize it's a live image, and infect your windows partition.
If none of that made sense, don't worry. Just do that other thing. I guarantee it'll make your computer last years longer than it otherwise would.
Also don't install any toolbars...