Computer slow and shuts down

Hello. Over time my computer has gotten slower and slower. Now it will just shut down every so often. I have checked it for dust and cleaned the inside the tower. But overall my computer just runs very badly. Startup takes forever and I get all kinds of error messages that I just close out to continue. I have to reboot quite a bit and then it will run slow but "ok" for a day or so then it slows down again.

Can you help me? I have an older computer that uses windows XP is it time for a new computer?

Please advise.

Thank you.