magentry..
You may want to try some of the suggestions in the below thread. It's somewhat similar to the thread Mark directed you to, but without the printer issues. It also references similar Windows Installer errors, along with what Microsoft suggests.
"Windows Installer and Error Codes 641 & 80070641 Issues"
'Something has happened to the windows installer on my computer. I have vista home premium 32 bit. I tried searching your site to download the installer but I could not find anything. I tried installing Java and it said it couldnt because the windows installer could not be accessed. and I have been trying to install some windows updates and keep getting error message codes 640 & 80070641. everything was working up until a few days ago when I tried updating nothing has been downloaded to my computer prior to errors.
http://answers.microsoft.com/en-us/windows/forum/windows_vista-windows_update/windows-installer-and-error-codes-641-80070641/24fd9f52-7faa-4f80-8f8e-8c7cd80dd481.
• When Mark asked if there were any strange happenings since WU last worked, you mentioned you were having problems deleting and updating software. Were you experiencing these problems PRIOR to the Windows Update issues? After? Or about the same time. As horrendous as I found FixCleaner to be, I'm left wondering if it might be a separate issue, and unrelated to Windows Updates. ONLY because of the Installer errors. 
• FixCleaner scares the heck out of me. And because it does, I would suggest scanning with Malwarebytes' Anti-Malware to be on the safe side. It's free. You have nothing to lose. And you never know what might turn up.
• Was it April 10th's updates you were having problems with? If so, the .NET Framework updates were included. They tend to be problematic. Microsoft offers a.NET Framework Repair Tool. As MS describes it, "it detects and tries to fix some frequently occurring issues with the setup of or with updates to the .NET Framework. Additionally, this tool tries to resolve these issues by applying known fixes or by repairing the installed product. The tool can also be used to repair corrupted Microsoft .NET Framework 4 installations."
I'm not suggesting using it, at this point in time. IF it turns out to be part of your problem, it may be easier to uninstall it, and install whichever version you need.
• Note in the thread "Error "The Windows Installer Service Could Not Be Accessed" when installing or updating programs on Windows 7 or Windows Vista". It's worth investigating, as one of the symptoms includes the error ""The Windows Installer Service could not be accessed. This can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance." Look familiar? (Solutions are included)
It should keep you busy for a while.
And hopefully, something will help. Or maybe, it will help Mark further help you. 
Carol