I don't use Norton or McAfee, I run AVS free, so I'm sure that's not the problem. I installed and ran Avast on reboot, but it didn't find any problems. However, once the reboot finished, the computer ran at a much better speed. Maybe something about the way Windows starts up causes the sluggishness? When I next restarted, however, the startup and operations were slow again. I also ran Ccleaner slim, and regained about 200 MB of hard drive space, but I have about 6Gb free, so I'm not sure that's the problem, either.
I also ran a Dell-specific startup diagnostic on the hardware. It found some issues with several "blocks" in the optical drive, and also that the processor fan was running at about 2000 RPM instead of 4000. I know overheating can slow things down, but the laptop doesn't feel overly warm.
2WiseGuys, how would I be able to check if blown capacitors were the problem? I'd love to be able to do some diagnostic work to isolate the issue, rather than installing and running software with a shot-in-the-dark approach. Thank you both for your help and suggestions.