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_9CABLE_DSL_FIBER_DIAL-UPWinXP_2K_CABLE_DSLMTU 1500Cable & 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?