Windows Installer errors

Newbie here! I've spent the last couple of weeks trying to fix windows installer
Windows 7 Ultimate SP1 3 bit - Windows Installer 5.0.7601.17807 modified 20/11/2010
Most tests run fine; updates are up-to-date. Scan by Microsoft security essentials OK; no action items or maintenance issues.
services.msc produces Windows could not start the windows installer service on local computer.Error 2: The system cannot find the file specified.
Attempt to install HP media smart server software produces "The windows installer service could not be accessed. This can occur if the windows installer is not correctly installed.."
sfc/scannow OK
command prompt MSIExec produces window containing what appears to be an intact windows installer 5.0
I was quite stumped until reading some threads here and accessing services where I noticed 4 old files (now in skydrive) from 18/3/2010 (when my installer was modified ironically)
msvcr71.dll
unicows.dll
uninstall
uninstall
On opening one of the uninstalls I noticed a number of errors beginning with "unable to startup...sorry the installer was unable to start up
I somehow got a prompt asking if I wanted to uninstall and figured wtf and got:

"The registry key "Blizzard Entertainment\world of warcraft trial\gamepath [\speed test; \install path] could not be removed.
Any help,suggestions,sympathy greatly appreciated

Yes

Work through the Troubleshooting steps in the link in my signature picture below.

Hi Gregrocker,
I think you are my new best friend! I can't thank you enough for providing such a thorough and comprehensive 'compendium'!!!
I'll admit I was feeling quite defeated and over whelmed as this is not my area of expertise. In my wading through the web of unknown, I was feeling that I may be just sinking myself further into a deep hole. I am only just through step 2 but my confidence has grown significantly. Thank you and this forum!! I was prepared to just trash this but I feel that I have gained a new found respect for this "non apple" device that I have cursed and lost sleep over these past couple of weeks!!

I purchased the Malwarebytes program (I would not have done except this forum I feel I can trust...thanks again) and ran the basic (non safe mode) this morning and was surprised to have about 225 'bugs'. I then followed your guide and ran the full monty in safe mode with my other new found friend, the hidden administrator.
Only 1 bug!

I'm onto system config and have a quick question. When you say ' Establish a Clean Boot by unchecking everything in msconfig>Startup except AV: Startup Programs .'

What do you mean by AV?

I have checked 'selective startup' and in 'startup tab' check disable all. Do I leave every thing enabled in the 'services tab' for the first go?

Thanks

AV is your antivirus. I´d use free lightweight Microsoft Security Essentials.

It tells how to achieve a Clean Boot in tutorial.

Malwarebytes is best used as a free on´demand scanner, however its Real Time protection is valuable if your prone to infection. But you still need MSE as an AV.

In your case I would proceed to the Clean Reinstall as so much infection is nearly impossible to overcome. Did you run the SFC yet to see how damaged System Files are?

OK, thanks. Is it OK to run both MSE and Malwarebytes?
sfc/scannow- Windows Resource Protection did not find any integrity violations.

Can I download an installer 5.0? Do you know anything about putting something (long command) into the registry? Is this OK to try?

Yes, the two provide slightly different types of protection.

I would not edit the registry.

If your prob persists in the new or Admin account, I would run the Repair Install or start fresh with a perfect Clean Reinstall

OK, thanks. Problem persists. I must have deleted the file (dll.cache?) I will proceed to clean reinstall with fingers crossed and your directions.
Since I have W7Ult SP1 I will download
Windows 7 Ultimate x86 SP1 U (media refresh)
X17-59463.iso

Read about some problems with media players, I have an extensive music collection and use HP mediasmart server and Windows media....should I be concerned?
Thanks so much, I'll report back when I'm done (may take a while tho'

Not familar with HP MediaSmart but dont use or recommend HP crapware. For files which WMP or WMC won't play I suggest VLC player at its defaults.