Lightweight Antivirus for Old PCs in 2026

An older PC doesn't have spare CPU or RAM to give up to a heavy background antivirus process — but it still needs real protection, arguably more than a new machine, since older OS versions can lag behind on patches.

What we measured

Boot-time impact, file-copy speed, and app-launch latency before and after installing each antivirus, isolating the products with the smallest measurable difference.

Our lightest picks

• ESET — the lightest footprint of anything we test, across every category.
• Windows Defender — zero extra install footprint since it's already part of Windows.
• Malwarebytes — real-time protection that stays light even with limited RAM.

See the full ranked list with justifications on Lightweight Antivirus.

A quick tip before you install

Uninstall any existing antivirus completely before installing a new one — running two real-time engines simultaneously is one of the most common causes of a suddenly-slow PC.