I do think it's very strange that it told me this month's Malicious Removal Tool successfully installed, yet did not run.
If you'd like to take another shot at running the September 2016 v5.40 version of the tool on your Vista machine, you can try the following:
Download the tool from the Microsoft Download Center at https://www.microsoft.com/en-us/download/malicious-software-removal-tool-details.aspx and save the latest Windows-KB890830-V5.40.exe file (32-bit) or Windows-KB890830-x64-V5.40.exe (64-bit) file to your Windows desktop. Close your browser, and then right-click the .exe file on the desktop and and choose Run as Administrator (a double-click might also work if you are logged into Windows with a user account that has Administrator rights) to launch the tool. When you browse to the download page it should auto-detect whether you have 32-bit or 64-bit Vista but just double-check that you are downloading the correct version.
MSRT Download Page Vista Sep 2016.png 22.15KB
There are quite a few Vista users (especially those using IE9) reporting that the Microsoft Download Center is behaving a bit wonky these days. If you are offered an old v5.39 version of the tool or have a problem with the download, try downloading from the MajorGeeks site at http://www.majorgeeks.com/files/details/microsoft_malicious_software_removal_tool.html - just be sure to use the appropriate 32-bit or 64-bit link to match your Vista OS. I don't normally recommend that users download Microsoft updates from third-party download sites but the links for the MSRT on MajorGeeks download directly from https://download.microsoft.com and I confirmed that the file size and SHA-256 hash of the downloaded Windows-KB890830-V5.40.exe files from the Microsoft Download Center and MajorGeeks are identical.
If you still have problems, see if you currently have the latest v5.40.13000.0 (or some other similar v5.40.xxxx.x) of the mrt.exe file at C:\Windows\System32\mrt.exe. If you already have the latest version of the MSRT in this location you can trigger a scan manually by following the instructions in the MalwareTips tutorial How To Use Windows Malicious Software Removal Tool.
Also, the other computer, which runs XP, has no record of even installing this month's removal tool, even though it had been doing so until last month.
I'm not sure if there was a recent policy change regarding Win XP SP3 and the MSRT, but I found a few posts by Win XP SP3 users in the DSL Reports forum reporting that they didn't receive their normal MSRT update for September 2016:
[XPHome] Windows Malicious Software Removal Tool no longer works on XP
Did MS stop updating its Windows Malicious Software Removal Tool?
Further to opera's suggestion to browse to https://www.microsoft.com/en-us/download/malicious-software-removal-tool-details.aspx using your XP machine, I noticed that the MajorGeeks download page for the MSRT also says "Requires: Win 10 / 8 / 7 / Vista".
32-bit Vista Home Premium SP2 * Firefox v49.0 * NIS v220.127.116.11 * MBAM Premium v2.2.1
Edited by lmacri, 22 September 2016 - 02:37 PM.