Error Code: 0x80070643

Hurray, it works !

I can't stand such essential updates remaining uninstalled, so I checked all
the forum for a solution. And talo's one did it (all the other ones failed).
Funny is, that I tried to repair MS Office 2003 earlier, using the CD, but
that didn't work (probably because I didn't change the register manually).
Question remains, why Microsoft doesn't give this procedure (otr another one
that does work) when one clicks the error code.

I was finally able to install microsoft security scan after receiving Error Code: 0x80070643 numerous times. I tried several things so here is what I did. Uninstall any previous antivirus, make sure Automatics updates is on, installed windows update Windows Installer KB893803

Hope this works for you as well