How do I know if my old laptop has malware?

Hey everyone! I have this old HP laptop that I mainly use for gaming and schoolwork. I've been downloading mods for The Sims from trusted sites like Curseforge and Patreon, but I've noticed that my computer becomes slow after using them, so I frequently remove them to free up space. Recently, I deleted the mods, the game, and the associated apps, and my laptop was running fine for a day. However, I then noticed a pop-up saying my computer was unprotected and suggested I download antivirus software, which I dismissed quickly. The next day, when I logged in, I was met with a black screen and couldn't access anything. I suspect it might be a memory issue or potentially malware, but since my laptop is quite old, I'm unsure. Any ideas on how I can figure this out and fix it?