Flavius: Thanks for all your help attempts!
OK, this has got to be the gnarliest problem I've ever seen!

I've tried all your suggestions, but to no avail. Not only can the 2005 C++ (8.0?) runtime not get its patch (so I disabled it in WU for now), but I can't install the 2008 (9.0?) redistributable either (original nor SP1). Both give 1935 errors (see below). What's odd is that it seems apps that call C++ redistributable services work, but there's no entry in Programs & Features that show either is installed. So, I'm inferring that both the 2005 and 2008 redistributables are part of the Win 7 O/S installation itself, since, looking in the %windir%\winsxs folder, I see x86_microsoft_vc80 and *_vc90 folders in there that I assume are the runtime "assemblies", despite there being no uninstall entries in P&F — unless those are remnants of old installations that somehow didn't get uninstalled.
BUT, at any rate, I was running Trend Micro Internet Security (malware's never gotten past the product), and they sent me a free upgrade to their new "Titanium" package. So, I removed TMIS to install the new one, and its MSI tries to install the VC++ 2005 redistributable — which fails again with error 1935 of course, so rolls back, and I can't install their antivirus package at all. To work around that and not have to run without an AV net, I re-enabled Windows' own firewall and Defender, then installed MS Security Essentials, which looks like it works great.
Anyway: I've looked in the installation logs for the runtimes, but they give no clear clue as to what's failing; nor does the cbs.log. Seems to me from search-engine browsing that these VC++ runtime installation problems are rampant, and NO ONE gives even reasonably technical help for them.
I wish I could find instructions for manually removing all files for, and Registry entries that pertain to, the runtimes (including stuff in winsxs). It should then be possible to reinstall them without collisions that result in 1935 errors, right?
Here're the error message for today's 2005 & 2008 runtime install attempts:
- Product: Microsoft Visual C++ 2005 Redistributable -- Error 1935.An error occurred during the installation of assembly 'policy.8.0.Microsoft.VC80.ATL,type="win32-policy",version="8.0.50727.762",publicKeyToken="1fc8b3b9a1e18e3b",processorArchitecture="x86"'. Please refer to Help and Support for more information. HRESULT: 0x80070003. assembly interface: IAssemblyCache, function: CreateAssemblyCacheItem, component: {66332652-9C28-58B1-A01F-C8B3B9A1E18E}
- Product: Microsoft Visual C++ 2008 Redistributable - x86 9.0.30729.17 -- Error 1935.An error occurred during the installation of assembly 'Microsoft.VC90.ATL,version="9.0.30729.1",publicKeyToken="1fc8b3b9a1e18e3b",processorArchitecture="x86",type="win32"'. Please refer to Help and Support for more information. HRESULT: 0x80070003. assembly interface: IAssemblyCache, function: CreateAssemblyCacheItem, component: {AE56AAF5-F3C0-3D4B-8859-A1E50A3E27BF}
Notice they both have the same HRESULT and publicKeyToken. "Assembly" seems to be jargon for the winsxs folders I refer to above, along with the most recent stuff in %windir%.
There are pretty verbose cbs.log sections for both failed redistributable attempts, which have a bunch of "Call to uninstall failed" errors in the transactions. There's nothing suspicious in the MSI Installer Cleanup list.
Any more ideas? I'm stumped. Thanks again.