Modern operating systems, or even older operating systems loading modern websites and software, will struggle with anything less than 2GB of RAM (4GB is a good baseline for a new computer). If all that turns up nothing, you may just be overdue for a new or more powerful computer.
So you may have to break out a rescue disk or system backup. Just know that if malware is deeply rooted in your OS, your computer may no longer boot once it's been eliminated. Short of that, run Malwarebytes Antimalware, a free program. If that turns up no errors, but things are still slow, you may want to try booting your system from a Kaspersky Rescue Disk, which can find and eliminate some of the stealthier viruses that circumvent Windows- or Mac OS-based antivirus software.
Your drive may be faulty and about to fail. If you've done all these things and your system is still slow, scan your drives for hardware errors using Disk Utility in Mac OS or CHDSK in Windows.
