Slow internet connection - Windows 7

I have a friend who had a virus issue on her computer that was solved through the intervention of the members here, and so she recommended this site to me when I ran into problems recently.

On Jan. 20, my computer's internet connection slowed considerably. After doing the usual (restart, check cables, ran system diagnostic), the problem persisted. I tried to do a system restore to a date prior to Jan. 20, but the procedure failed -- I got the following message: "An unspecified error occurred during system restore (0xc000022)." I ran Malwarebytes and it identified a few spyware programs, which I deleted -- subsequent Malwarebytes scans reveal no additional problems -- but that hasn't solved the problem. I ran a pingtest, which gave me a B grade.

Here are the facts:
I have a DSL connection and an external modem. I'm running System 7 on an HP e9210t.
I'm using Firefox 9.0.1, but the problem also exists on Internet Explorer.
I do an automatic disk defragmentation every week; my hard disk is currently 0% fragmented.
The computer itself has no performance issues. All non-internet-related programs work fine.
I've never had any problems like this in the past, and I don't think there's a throttling issue with the ISP.

I don't have cable tv and broadcast reception around here ranges from fair to abysmal, so I do about 98% of my television viewing on my computer -- not to mention streaming videos on Netflix. I have about two dozen episodes sitting on my Hulu queue and the new season of Archer just premiered, so this is starting to be a real quality of life issue for me. Any help that you can provide would be greatly appreciated.

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