There are so many ways to try and diagnose these things.
Some may suggest updating all your drivers first, but to be honest, if I were you I'd seriously consider doing a fresh install of windows just to completely clean your system out.
After a few years of using it, I imagine there are a fair number of resource suckers hidden in your computer. But before you do a reinstall, a couple of questions:
What version of windows are you using?
Do you have the instal disk?
Do you have disks for all your software?
Can you do a back up of all your important data (my documents, saved games, videos etc.)
Do you have all your drivers for your hardware?
If the answer is no to any of these, then you probably shouldn't do a reinstall, instead to a windows REPAIR (I am assuming you are on XP).
But before doing that, I would run a test on your memory to make sure it is still up to par. Download and run
www.memtest.org and run it for a few hours.
If you want to skip all of the above, and try another possibility, then let's just go ahead and update your drivers.
We need the details of your computer in order to help you find your drivers. What make/model is your motherboard? Is your computer a brand name like a Dell or something? If so, what model. If your system is NOT a brand name, and you don't know what parts you have inside, download and install
https://www.techspot.com/downloads/433-everest-home-edition.html software and tell us what it says about:
motherboard
memory
graphics card
sound card
Also, open up your case, and look at the sticker on your power supply. What make/model is it? How many watts is it rated at?