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My PC is acting very oddly: What should I do?

I received a plaintive message from a desperate reader seeking advice which prompted me to write this piece. This reader's problem was an odd one, as his computer was exhibiting a wide and weird variety of bizarre symptoms. Programs shut down spontaneously, windows scrolled up and down, the mouse pointer disappeared, and simple browser tasks like choosing a tab didn't work correctly. Despite this circus of bizarre behaviours the installed antivirus software reported no malware problems.

I'm not psychic, and I couldn't deduce the source of the problem from the list of symptoms. However, in this or any situation involving unexplained activity like this user experienced, I'm inclined to blame some application that's running in the background. Here's how to determine which application is the cause of the trouble.

Start by launching the System Configuration utility (MSCONFIG). In Windows 8.1, Windows 7 and Windows Vista, click the Start button and simply enter MSCONFIG. Click the Boot tab, check the Safe boot box, and choose the Network option. For Windows XP, click Start, click Run, and enter MSCONFIG. Click the BOOT.INI tab, check the /SAFEBOOT box, and choose the network option. Now click OK and reboot the system into Safe Mode.

Safe Mode is ugly. Windows starts with only the bare minimum of services and uses a lowest common denominator display mode. Ignore the ugly display and try to recreate the situations in which the bizarre behaviour appeared. If it still happens in Safe Mode the chances are good that the problem is hardware-related. In that case you're going to need help from a technician. But if the problem seems to be gone the next step is to find the culprit application.

First you'll need to reconfigure Windows so it doesn't start in Safe Mode. Launch MSCONFIG as before, click Normal Startup on the General tab, click OK, and reboot. Whew! It certainly looks better now.

Launch MSCONFIG once more, and click the Startup tab (in Windows 8, the Startup tab can be found under the Task Manager, which you reach by pressing Control-Alt-Delete together, as ever).

This tab lists all non-essential programs that launch at startup, with a checkbox to reversibly disable each item (or a Disable button in Windows 8). Disable all of the startup items, click OK, and reboot. One warning: The adjacent Services tab looks similar to the Startup tab, but you must not wildly disable services. Doing so could render Windows non-functional.

If the problem is gone after rebooting, you've confirmed that one of the startup programs was the cause. To determine which one it was, use MSCONFIG to re-enable the startup items one at a time, rebooting after each change. When you've identified the culprit, re-enable all of the other startup items.

You now have a choice. If the problem program is unimportant, uninstall it. If it's of occasional use, leave it disabled at startup and launch it as needed. Yes, you'll get the problem back along with the program, but rebooting will cure that. If you really need the offending program, seek an update or contact the software maker’s tech support.

It is, of course, entirely possible that this exercise will not solve the computer's bizarre behaviour. But it's something you can do for yourself without taking your PC to a repair shop or paying a technician to come out and look at it.