Windows 7: Cannot Install Microsoft Security Essential, error 0x80070645

??? what does it look like to you

Send you a P.M.

It's fixed.

tried it, still the same

This is a option. Take a good read through this and see if it is what you want.
Repair Install

You could try running an SFC & see if that finds any problems

SFC /SCANNOW Command - System File Checker

Also, here is a MS Fix it tool for uninstalling MSE. You might want to try that & see if MSE installs after running this.

http://techdows.com/2010/11/microsof...-computer.html

Okay top secret now would be a good time to make a list of what you have tried and what the out comes were. We are in the guessing stage until we have that information. You posted # 5 doesn't tell us what you tried.
Yes, I did. I have searched Microsoft web site, tried many solutions but did not work.

I'm going to ask for two logs .... CKScanner and Security Check.
***NOTE: Run both of these apps as 'Administrator' by right clicking and choose to run as Administrator from the dropdown menu.

Download CKScanner by askey127 from HERE
Important - Save it to your desktop.
Doubleclick CKScanner.exe and click Search For Files.
After a very short time, when the cursor hourglass disappears, click Save List To File.
A message box will verify the file saved.
Double-click the CKFiles.txt icon on your desktop and copy/paste the contents in your next reply.

and

Download Security Check by screen317 from here http://screen317.spywareinfoforum.org/SecurityCheck.exe or here http://screen317.spywareinfoforum.org/
Save it to your Desktop.
Double click SecurityCheck.exe and follow the onscreen instructions inside of the black box.
A Notepad document should open automatically called checkup.txt.
Please post the contents of that document.

For Seven :
Remove registry key :
HKEY_CLASSES_ROOT > Installer > UpgradeCodes >26D13F39948E1D546B0106B5539504D9
For Windows XP :
HKEY_CLASSES_ROOT > Installer > UpgradeCodes > 1F69ACF0D1CF2B7418F292F0E05EC20B
And Install it !

Cheers Gabard... although the Registry Keys were the other way around for me, I found the 26D13F..etc on the XP machine, deleting this allowed me to install so kudos.