More updates on the problem (see the last three paragraphs):
So, to recap:
In IE6, on WinXPProSP2, with all updates, I suddenly cannot download any
file, despite putting the file location's URL into Trusted Sites and setting
the Security Level to default (Low) or even Custom (enable turned on where
Low did not).
Sites affected:
http://www.apple.com/quicktime/download/win.html ,
http://www.majorgeeks.com ,
http://www.download.com , etc.
The issue is IE6-specific, and downloads in Firefox 1.5.0.7 are not affected
(thankfully).
The error message given is this string's title.
Antispyware scans are all negative: Ewido, Spybot Search & Destroy,
Ad-Aware, and Windows Defender.
Antivirus (and cross-product) scans are all negative: Windows OneCare, AVG
Free 7, Panda Online ActiveScan.
Windows Firewall has been turned OFF and ON, without affecting the problem.
Windows Defender has been uninstalled.
HiJackThis did not turn up any malicious nonsense.
The only BHOs installed are Adobe Reader, Spybot, and Google Toolbar.
No other antivirus or firewall product is, or has been, installed or removed
(which means none of the following: ZoneAlarm, Norton Internet Security,
Norton Firewall, Norton Antivirus, Mcafee, TrendMicro, Panda, etc.).
No software has (deliberately) been installed since September 1 (long before
the problem appeared). System Restore to September 1 does not fix the issue.
Dial-a-fix (from DjLizard.net) did not correct the problem, even after
running all standard repairs and then doing a reinstall of IE using files
from an XP install CD.
I used SFC /PURGECACHE and SFC /SCANNOW to see if something was corrupted.
Many CD-accesses later, it finished, but the downloading problem remains.
Then, I uninstalled Windows OneCare. Before rebooting, Windows Firewall was
turned OFF, with the message "For you security, some settings are controlled
by Group Policy". However, Windows Firewall was turned back ON after reboot,
so I guess that was just part of the OneCare uninstall. (It looked weird,
though, and if this is normal, it should be mentioned before the uninstall
occurs.) Then I tried to test downloads again. ACCKK! Now I could not
even get to the browser's home page. A quick check confirmed: the network
card (Intel(R) PRO/100 VE) was missing from Network Connections, and in
Device Manager it said it could not start (Code 10). Yuck. Uninstalling and
reinstalling it did not help. I quickly attached a USB-to-Ethernet adapter
(I have several), and I was back on the Internet, but not before Windows XP
crashed (hard reboot), right after clicking on IE6 to try the new connection.
After the reboot, Firefox worked normally, but IE6 is still sick.
I am becoming convinced that I have a rootkit, and for some reason, it seems
to be trying to annoy me (instead of hiding). Since only my
less-than-computer-literate-by-choice wife has physical access to my
computer, it seems likely that it's "something it ate". I am going to try a
few more things, but it seems likely that I'll have to blow out this drive
and reinstall. Ugh.