This could be any number of things so there is no "simple and quick" solution.
First, when you say it is slow is it slow to do ANYTHING or just downloading things from the web?
If it is just the web then maybe your broadband is slow. Have you checked if other devices that use the web (tablet or smartphone or other computer) are also slow.
If it is not just the web but everything that is slow then as I said, it really could be anything.
You may have too little RAM memory, the hard disk may be full up, there may be a fault on one or other of the components (hard disk for example).
There may be a bad performing program on that is running all the time and use up all the computer power.
Also there may be a virus or malware (malicious software) on it.
Without sitting in front of it I cant really help you fix it.
Do you know anyone that knows a bit about computers who could have look at it for you?
Often with an old computer like this a complete reinstall of Windows is one sure way to get rid of all the rubbish on it and speed it up. But this is no quick or simple solution for somebody who has never done it before.