I have multiple winlogon.exe files on my machine, and one's taking up a lot of memory - what do I do?

The short version is that winlogon.exe is the process that handles your logging in to Windows. When you click on a user name after boot up, or enter a user name and password, that's winlogon handling the job. It also handles logging off and a number of other things, but you get the idea; its name actually does a good job of identifying its primary function.

Now, because it's always present, the name "winlogon.exe" is a favorite target for misuse by malware authors. By distributing their bogus programs with a name of winlogon.exe things appear "normal" when the casual observer looks at a process list using task manager.

Now some folks will tell you that if you see a winlogon.exe anywhere other than c:\windows\system32, that the other copy is a virus. Not true. Given the way that Windows Update works, and the way that Windows File Protection works there may in fact be several copies of winlogon.exe that are perfectly valid, and possibly not even the same version.

For example, here's what's on my Windows XP Pro machine:

• C:\WINDOWS\SYSTEM32\winlogon.exe: 502,272 bytes dated 2004-08-04 00:56:58. This is the "real" version of winlogon.exe that's actually running on my machine.

• C:\WINDOWS\SYSTEM32\DLLCACHE\winlogon.exe: also 502,272 bytes and also dated 2004-08-04 00:56:58. This is the copy used by Windows File Protection - should the "real" winlogon.exe in SYSTEM32, above, become corrupt or be overwritten by another, WFP will replace it with this master backup copy. (Should this backup copy become corrupt or disappear, I believe that WFP will then ask for the installation CD instead.)

• C:\WINDOWS\ServicePackFiles\i386\winlogon.exe: still 502,272 bytes and still dated 2004-08-04 00:56:58. This is more-or-less the equivalent of the C:\I386 folder, which typically contains a copy of the files from your installation CD, except that this is a copy of the files which were updated in Service Pack 2. I believe it's used by WFP if the DLL Cache doesn't work for some reason. ServicePackFiles\i386 is also used (like C:\I386) if new components are installed that require additional operating system files that weren't already installed.

• C:\WINDOWS\$NtServicePackUninstall$\winlogon.exe: this time we see a size of 516,608 bytes and a date of 2002-08-29 03:00:00. This is the version of winlogon.exe that was replaced by Service Pack 2, and which is preserved in case SP2 is ever uninstalled.

• C:\I386\winlogon.exe: ok, this isn't on my machine because I moved my C:\I386 directory to another location on my network, but you may well find it here.

All of those are valid, and their presence does not indicate that you have malware.

However, if you find winlogon.exe anywhere else on your machine ... well, then perhaps it's time for a little concern followed by an up-to-date virus scan.

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To examine what's running on your machine, I'd recommend using Process Explorer rather than Task Manager. We can get a little more information out of procexp.

With Process Explorer running, just hovering over the line for winlogon.exe will show perhaps the most interesting bit of information of all:

As you can see the popup tool tip shows that this instance is running from the copy of winlogon.exe in C:\WINDOWS\SYSTEM32. In other words, it's running the copy that we expect it too. If not, then it's time for that up-to-date virus scan.

Right click on winlogon.exe in Process Explorer and select Properties and you'll get the same information and some more details:

Click on the Performance tab and you'll get some information about winlogon's resource utilization:

Most interesting might be the Virtual Size (199,168 K on my machine, or around 200 megabytes), and the Working Set (22,516 K). This is a real example and I'd expect them to be pretty normal and representative numbers.

So, what if your numbers are way off from that? Or what if your legitimate instance of winlogon.exe in C:\Windows\System32 is eating up all your CPU? Turns out there are several different potential causes. Searching Microsoft's support site for winlogon.exe returns several articles describing several different scenarios. They're rare, but if you've determined that you're not fighting a malware infection, they're the next place I'd look for what to do next.