I've Resolved This Issue On My Windows 7, 64-Bit Computer
For me, the source of the problem was the Indexing service. There were some issues with disabling indexing, though. Simply disabling the service in the Adminstrator's Tools didn't prevent files from being indexed... at least from my understanding of the situation.
I actually had to open up My Computer, right-click on my HDD's and select "Properties." From there, there was a box checked which reads, "Allow files on this drive to have contents indexed in addition to file properties."
After clicking "Apply," and waiting a while for the properties of all the files on my HDD's to be changed, the lagging immediately ceased. Apparently, there was still some type of "indexing" going on, even with the service disabled.
I did this about 2 days ago, and since then I haven't had a single issue of lagging. Normally, it would happen every minute or so, especially while watching YouTube videos or listening to iTunes.
It appears that Windows 7's indexing service is a huge resource hog.