You missed he most obvious answer - RAM.
You didn't say how much ya got, I don't need ta know either, one things fer sure, it ain't enough.
All that Anti Virus, Reg. cleanin' and whatever else ya got runnin' only adds to the amount of RAM yer using up if ya got anything runnin' in the background.
Upgrade to at least 2 Gigs of RAM ta give ya peece o' mind.
The reason it don't happen when the CPU is above 50% is cuz the background stuff is done fiddlin' around.
Ya dinna give no info, so if all else is 'up-to-snuff', ya should be OK.
If ya got a PC with crap fer RAM, yer PC probably ain't the newest, mebba it's time to think about upgradin' fer real.
;-)