I am running Windows 7 64 bit. Whenever I try to do anything in Explorer (navigate pages, open windows, etc) it locks up for a few seconds, sometimes a minute before I can do anything again.

Resource monitor shows good CPU and memory stats but constant hard disk usage spikes.

I have run Microsoft Security Essentials, Malwarebytes and Checkdisk. It has found no viruses, malware or bad hard drive sectors.

I have run HD Tune Pro and have two warnings: (05) Reallocated sector count and (C4) reallocated event count but it passes the SMART test and other basic tests.

And the weirdest part of all: when I play a video in Windows Media Player, the system runs almost seamlessly.

What is going on?