computer running slow even though 15% memory remaining

Sometime I do somewhat intensive processing that causes 85% of my 6GB ram to be used. But when 85% is being used, my machine completely slows down for all other tasks including very basic tasks like opening Windows Explorer. Surely the remaing 15% should be more than enough to open Windows Explorer without any latency. I'm using 64-bit Windows 7 Ultimate. Also, I should state that the CPU usage is very low at these times.

Thanks so much.

The hard drive is at about 75%. It only slows down when the memory gets above 80% so I know that it's a memory issue.

Win7 should never hang or bog with a perfect clean install and adequate hardware.

To do the first look at this: Clean Reinstall - Factory OEM Windows 7. The steps for retail are the same.

Until then you can Clean Up Factory Bloatware - Windows 7 Forums.

Test your RAM - Test with Memtest86+ for 5-6 passes or overnight to stress it. Post back your specs so we can see the amount of RAM, model Processor and whether you're running 32- or 64-bit Win7. You may need to upgrade RAM and can start by googling your computer model RAM Upgrade and running the Crucial System Scanner software to find out what type of memory is in your computer