Wow, thanks for the speedy reply.
I believe, IIRC, that both of the hard drives are 40GB (the faster computer has 25GB free, while the slower one only has 13GB free). And they both have roughly the same stuff installed on them.
I totally removed what they didn't want using Revo Uninstaller, including Adobe, and gave them FoxIt instead.
The slower computer was purchased at a local Gateway store circa 2003, which eventually closed due to bad local press. They admitted to using cheap parts in building their computers and basically cutting corners where they could in order to maximize their profit margins. Please note that this is not a slam against Gateway in general, just what the manager of this particular Gateway store confessed to eventually. So it likely is not a reflection of Gateway in general.
Given this info, I am somewhat surprised the PC is still in as great of shape as it is, however, it very well could have been purchased before the problems starting at this particular outlet, since I believe the media jumped on it sometime in 2004.
I used Black Viper's "safe" recommendations for their services.msc at Start > run. And I gave them both Avira for an anti-virus, with realtime protection (that does take up quite a bit of memory, it seems). And I gave them both the free version of Malwarebytes Anti-Malware.
I ran OTListIt and Rooter on both computers and see no rogue drivers/files or inappropriate entries in the registry dump. And everything the anti-virus or anti-malware picks up is minor, like coupons.com garbage, nothing as serious as Vundo and certainly nothing as damaging as Virut, so I am thinking the slow-down on the one computer, despite the same amount of RAM, is not due to viruses or spyware, but just that one has better specs. Plus, as I noted above, one has much more HD space.
Less HD space can seriously slow down a computer? I always thought the slow-down was minor, but perhaps I'm wrong.
For example, on MY OWN computer I have a 150GB HD and only 3.5GB free. That's awful isn't it? Hopefully it won't damage the computer. I just need to get an external HD and spend a few days transferring files, to free up the internal HD. I just keep putting it off.
I treat my computer worse than I would ever treat someone else's!