I can totally relate. I spent two whole days on my (NEW) PC trying to figure out how to correct the same problem... "windows installer could not be accessed...error code 643) !! It can also come up with different error codes. anyhow, to cut to the chase, I ran accross this solution somewhere online in a forum on Windows and IT WORKED!!!! Try it. I don't know exactly how this guy figured this out but it definitely fixes the problem. From all the replies he was getting, it worked for a bunch of people having the same problem!

1) At Start type: regedit
Lft Click on: HKEY_LOCAL_MACHINE
Lft Click on: SYSTEM
Lft Click on: Services
Lft Click on: MSIServer

Rt Click on: WOW64, select Modify and change
value data from 1 to 0 (zero)

2) At Start type: run
Type: services.msc
Go to Windows Installer and stop and start the program.

3) Reboot your computer.

Now go and try and do an install. In my case the steps above took all of 3 minutes and i went to the application .exe file I was trying to install and it installed perfectly, not skipping a beat...everything since then has installed perfectly!
I'm curious to know if this works for you. Please let me know OK. Margie mp.baltierra@comcast.net

mpbaltierra post.

First of all I would like to thank mpbaltierra post. I am using vista home premium 64bit. I had update installer errors.... 643 windows installer cannot be acessed. Tried for days to install new version of windows live and windows updates. Searched and googled nothing worked.
Found this post tried it. Followed the steps and in 5 minutes everything began to install for me. Thanks again Marge.

Hi Gary...I am so pleased that this worked for you also. I'm not a geek or a techie or an expert but I am very good about researching and repairing my own pc problems and even done a few hardware repairs like installing a NIC, changing the inverter on my laptop's monitor when it seemed to have "burned out"...But that 643 error had me stumped. I struggled with it for 3 days. I wanted everything running correctly so I could install Windows 7 on my brand new desktop. So you can imagine HOW HAPPY I WAS, when I found that solution. God bless the clever person who figured out the solution!!! I would still be sitting here trying to find a fix had it not been for him/her!! Well anyhow, I'm so excited that it worked for you too. Happy Trails...
Margie/SF-Calif