When running Win10 on my laptop, using any security software, Avast!, Win Defender, causes constant 100% disk usage. As soon as I turn off RTS with Defender, it jumps down to 0. This isn't my main PC, do I need an antivirus if I only download from reputable sites? And any ideas how to fix this? I'm doing an SFC scan right now.
You should not run two anti-malware applications simultaneously as they will interfere with each other (though you can do a one-time scan of specific files with a second tool, such as Malwarebytes) . Avast! should have disabled the Windows Defender engine. If it did not, uninstall Avast! and then reinstall it.
Though Avast! is an excellent tool, you might try Avira or other free AV suite if you still experience the issue.