OK, I'm clutching at straws a bit, here.
I admit I don't know what is at play, and am guessing at possible solutions. (In no particular order.)
1) The original install file you have - or the installation itself- is corrupt.
A full reinstall from scratch may fix this.
-Get the uninstaller
here. (Also note the instructions on that page.)
-Get the full Avast install download, 36.99Mb, from
here, just below the words "Free registration" on that page.
- Go offline, uninstall Avast from the control panel. You would have to turn off the self protection module to do that. It can be done by right-clicking on the tray icon>program settings>troubleshooting. If not possible, proceed to the next step.
- Run the uninstall utility, in safe, as directed in the first link.
- Reboot into normal.
- Install Avast from the fresh installer file. Reboot as prompted. Go online, get updates.
2) When Avast was originally installed Vista's UAC was enabled, and prior to removing it to install MSE (or at some stage following that) you have disabled UAC.
I'm not a native Vista speaker/user, so don't know the full ramifications of that, bit there could, in such a case, be "lingering" files whether from MSE or Avast in a hidden folder somewhere.
If that was the case, it may work to re-enable UAC. Further investigation indicated (preferably from someone who knows about these things) if that could be a factor.
3) There is malware on the computer. To help eliminate that possibility (and the malware, if present) try
MBAM, install, update, run a quick scan, have it fix anything found. If malware is found that prompts for a reboot to complete the removal, do so promptly, post the scan report (if applicable.)
4) MSE has left some driver/dll/registry entries behind that are affecting Avast. I couldn't locate a specific uninstall utility for MSE; a manual search for folders/files/reg entries (if you are up for it) might reveal something.
A better way to proceed may be to remove Avast completely (using the removal steps above), re-install MSE, download
Revo Uninstaller and use it to uninstall MSE. A word of caution if you get this far and do this: Revo runs the programs' own uninstaller, then searches for files/reg entries depending on the level of "clean" you've selected. (Select deep.) Do not restart when prompted after the uninstaller has run. close that prompt (if present) and go to "next" in the Revo program. When it gets to reg entries, expand each flagged entry (where present) right down, and
only delete the ones in bold.
Hope that helps.