Alright so, as the stickies say, I'll post all the requested information:

OS: Windows XP SP2 (Media Center Edition)
ISP: Comcast (I believe I'm subscribed to the 6Mbps tier)

I used VanBuren's cablenut settings (and restarted afterwards) with no difference in speed. More specifically, this file:
VanBurens_cablenut_settings_version_9\CABLE_DSL_FIBER_DIAL-UP\WinXP_2K_CABLE_DSL\MTU 1500\Cable & DSL 6000.ccs

Only protocols installed on my NIC are Client for Microsoft Networks and Internet Protocol (TCP/IP).

I was connected through a router (with the latest firmware installed). But that was the first thing I suspected to be the problem, so at the moment I'm connected directly to the modem. Internet speeds are still just as slow.

My computer is, to the best of my knowledge, spyware/adware/malware/virus free. I have all Windows Updates installed etc. etc.

My speed with all testing websites was consistently this in November '07: http://img252.imageshack.us/img252/4903/204598111ek0.png
Compared to my speed now (again, on all testing websites and consistent): http://img248.imageshack.us/img248/457/241039308se9.png

As far as I know, we're subscribed to the same Comcast tier. Recently my mom cancelled the Comcast TV and phone services since we all have cell phones now. And we have DISH Network for our TV service now (mom wanted to save some money and failed miserably).

Sooo. Any ideas?

My Comcast speeds don't suck as badly as yours, but compare my November '07 test (in my signature) to what I get now:

I wish you all the luck in the world trying to get Comcrap to fix anything. They will send a "technician" out who will say you have a clear signal coming in and you are getting the best you can get. The fact that you were getting good speeds is completely irrelevant. If you press the matter hard enough, the "technician" can make a request to have a "line technician" come out to check the signal coming into your building (house, apartment, whatever). The "line technician" does not enter the building and unless you actually see this imaginary friend of the "technician's" you will not know he was there, nor will Comcrap bother to give you a follow up call after his alleged "service call". All you will notice is that nothing has changed and Comcrap will say that they can schedule another "technician" for yet another service call, usually for the most inconvenient time for you, the customer. Who do they think will be home on a Wednesday between noon and 5 P.M. anyway?

And yes, I have played this game a few times. Comcrap always wins because they are the only cable internet provider in the city of Houston, Tx. 

it could be you got dropped down to the lowest tier because of cancelling TV service. 6 meg tier is only available if you have cable tv AND internet subscription.  just my guess but that;s how Comcast is in my area (Pa).  in the process of dropping you down they probably hit the spite button