Uninstall/Install broken.... Where do I start debugging?

this is a new one on me. have not encountered this on Win7 before.
the patient is my trusty lab mule, which I put through all sorts of torture in solving other problems for other people - now its not well.

the system is a HP dc-series, Windows 7 Ultimate x64 - typical rig with a spinning SATA drive and 4gb ram, ati card.

the symptom here was that I noticed some longish lag in bootup after login - so, login, then get the pretty blues, and a too-long wait. so I went to the system error logs to see if anything left a trail. There I see that Bonjour services was failing/retrying/failing at bootup. I thought 'simple... i'll uninstall it, then reinstall'.
It would not uninstall -failed with something like "cannot find specified file".
so I tried d/l'ing itunes... thought I'd reinstall the basic apple pile of stuff, then uninstall if need be. Would not install: failed with same message.

so then I tried something unrelated: downloaded a simple utility I have used previously [non-apple, small, simple]: ran the msi and it too failed - same message.

then I tried general Windows Updates. had the usual pile of them there notified, winnowed the list and picked off 25 small ones. 23 installed ok, 2 failed with a Error Code 643.

I am suspicious that this all started with my stubborn repeat attempts to get .NET 4.0 to install - never succeeded. Read the very long threads on this subject on MS sites, tried all the fixes, nothing works. In the process I ran the .NET cleanup utility after each failure, before retries. none of that helped with the install.
Is it possible that some core windows functions that are needed for proper Installs got damaged, so file locations are not what are specified/defaulted?

all help greatly and thankfully received.

nuke the drive and do a clean install

or

pull hair out over .NET install/update issues

Have you tried Error codes

I recently fought a .NET update issue where the update process created the typical temp folder and then could not find it. I had to manually point the installer to each MSI.

i tried the msfixit for install/uninstall issues and it can do one at a time, but has not resolved the underlying issue.
it feels like a permissions issue: even if I run "as administrator" an install, it is not able to fetch/run its programs from the Temp directory /users/myname/Appdata
I checked each one, and it successfully creates the folder under Temp, but can't get to it.

?

As long as you turn Bonjour and all other freeloaders off at Startup and in msconfig>Services (after Hiding all MS) then it shouldn't interfere, although you can delete it's reg keys by doing a Search on regedit Edit tab, then delete the Bonjour Program folder to root it out.

We've found that most of the useless Bonjour will come out with the Program folder and a Reg search by it's name. Set a Restore point first.

I agree you may be overdue for a Clean Reinstall - Factory OEM Windows 7 for a trusty workhorse that deserves the best. If not work through the other Troubleshooting Steps for Windows 7.

what I was saying is that the Microsoft-provided "fixit" automaton for 'uninstall/install problems' managed to uninstall Bonjour, but the larger problem remains, which is that in general, installs and uninstalls alike fail because the installer cannot access the folders/items it creates in the user's temp directory, though they are all there [I check each time].

I ran sfc /scannow for windows 7 errors and it came up 100% clean.

I'm trying to learn something here but so far I'm not getting the message.

Try the Repair Install as a last resort before the clean reinstall.

zapp22 said:

....but the larger problem remains, which is that in general, installs and uninstalls alike fail because the installer cannot access the folders/items it creates in the user's temp directory, though they are all there [I check each time].....

Is this during a Windows Update install or did you download/use the standalone installer for the patches that would not install?

I get it now - you want to play around with this problem a while longer and learn what you can from it - you are not quite ready to just fix it. Okay, then to answer "Where do I start debugging?":
Downloading the standalone installer for one patch
Start Process Monitor v3.03
Filter on the standalone installer

Let us know what you find.