I've been experiencing occasional hangs over the past week or so where the computer becomes extremely sluggish. I'd try to bring up Task Manager to see what was running, but it took a good 2 minutes for TM to come up and by the time it did, the offending process had done its work and would not give itself away on the task list.

When it happened during a mix session I got fed up, dropped everything and set out on a search-and-destroy mission to find the culprit and terminate with extreme prejudice. It turned out to be a process named MsMpEng.exe and it was eating up nearly 100% of my CPU. Since disabling the process the hangs have not returned.

MsMpEng.exe is a component of Microsoft Security Essentials, specifically the "real-time protection" piece. In the Services list, it's called "Microsoft Antimalware Service". You can either stop it from the Services applet or from the MSE settings (click on "Real-time protection" and uncheck the "Turn on real-time protection" option).

This is why a DAW should just be a DAW and nothing else.

that's odd.  I've never had a problem with MS SE.

See This  http://social.technet.microsoft.com/Forums/en-US/Forefrontclientgeneral/thread/8ee4a53e-2ae7-4e91-9ac9-173ee3b12650

They talk about Windows Defender in that thread.  If you have MS Sec Essentials running you do not need Windows Defender running and it should be turned off. Conflicts with multiple security programs can cause a lot of problems. 
Running a security scan in the background along with other applications can also cause this.  You should set the scan time for an idle period (I have mine set to run weekly at an evening hour).
All of the reviews/reports I've seen on MS Sec Essentials says it runs well and is not intrusive like some security software. 

Hear, Hear... !

A DAW should be a DAW and nothing else. 

On all my online computers, I have been using MSE for over a year now with no obvious issues. 

See This http://social.technet.mic...4e91-9ac9-173ee3b12650 They talk about Windows Defender in that thread. If you have MS Sec Essentials running you do not need Windows Defender running and it should be turned off.

I don't have Windows Defender running. Apparently that functionality has been integrated into Microsoft Security Essentials. Same exe, same service, different name.

This behavior just recently started. I believe it coincided with the last Windows Update. I do not allow automatic updates, preferring instead to run the updater manually about every 2 weeks. I do recall that the last update included something about anti-malware but I foolishly paid it no mind, assuming it was part of the virus scanner - which I also run manually.

Bit, was there a specific time it happened (during the scheduled scan) or was it just random? 
I have MSE scheduled to scan when I'm not on my DAW, but definitely don't want something like this to happen (without me invoking it) while I am on the DAW. (If so, I may just need to disable "Real-time protection" while I'm on).  

On my other Win7 64bit system I had a similar problem.  MS SE (without Windows Defender) ran fine for a year or so but then it started causing delays in the system even though the task manager did *not* show high cpu usage.  I discovered that if I momentarily disabled MS SE then the problem went away for some number of day (can't recall how many).  That Win7 system does not have a DAW on it but the most obvious symptom was that you couldn't play a wave or mp3 file or play a video (like You-Tube) without lots of stuttering.

I finally gave up and installed avast! free.  No problems since then.

I noticed that when I uninstalled MS SE, Windows Defender showed up.  It seems that avast! and Defender get along just fine.

--Larry

was there a specific time it happened

I had the scheduled scan set for 2:00 AM Sunday morning, so it wasn't part of a scheduled scan. Like you, I don't let my computer do anything automatically such as virus scans or automatic updates.

It's possible that MS Security was locked in a battle with offending software (malware).  I just spent 3 days trying to get rid of a highjack program.  Managed to do that but then, in a couple days, my computer would freak out again.  Turns out the malware was installing a nasty little package called a "rootkit".  This fine piece of work was cloaked in my registry.  It would reinstall the malware.  It also created weird redirects in my Internet Explorer software.  I would key in "guitar strings" in the google window and the search would come up.  When I picked "earnie ball" I would wind up at some never-before-heard-of "search engine" site.
 
I finally had to boot from a Win7 install DVD and run system restore to set my machine back about a week.  Problems solved.  Well, knock on wood, anyway.

I've been running Avast! free in gaming mode for two years and never had a problem. I don't trust Microsoft to stay out of my busisness. I never use Internet Explorer, most viruses are written to exploit it's specific weaknesses. I run Malwarebytes periodically to catch the trojans. Never had an issue even with internet on.