For the last couple months I've been troubleshooting a vexing problem on my new quad-core HP desktop: Roughly once per week, the machine would start running as slow as molasses.

And I mean slow: Programs would take several minutes to load, and even simple activities like switching browser tabs were maddeningly sluggish. Curiously, this behavior would last for several hours, and then the system would be back to normal.

Knowing a thing or two about PCs, I tried all kinds of remedies. I checked for viruses and spyware. I ran memory and hard-drive tests. I uninstalled a few potentially culpable (or so I thought) programs, including HP's junkware. I even disabled a few Windows services (like Search and SuperFetch), which some fellow users suggested could be to blame.

Nothing worked. I even went so far as to wipe the hard drive and reload Windows (Vista x64, in case you were wondering), even though the system was barely a month old. That did the trick--for about a week.

This got so irksome that I started venting about it on Facebook. And that's when an old high-school buddy (you rock, Scooter!) came to the rescue. He suggested I check Windows' Disk Defragmenter. Sure enough, it was set to Run on a schedule, and the last time it ran matched up with the most recent slowdown.

Suddenly it all made sense: The non-stop hard-disk activity, the out-of-the-blue recurrence of the slowdowns, and the eventual return to normal. The only thing I couldn't figure out was why I never spotted Disk Defragmenter when I was poking around Task Manager. It was as though it was running so far behind the scenes as to be invisible.

I've since disabled the feature, and to date I haven't had a single slowdown. Granted, it's barely been two weeks, but I'm 98% sure this was the root of the problem.

What about keeping my hard drive defragmented? I can do that manually, once a month or so, on my schedule. In the meantime, where do I file a bug report with Microsoft? I suspect this might be a glitch in the 64-bit version of Vista. Let me know if you've encountered similar behavior and, if so, whether you were able to resolve it.