Command prompt is displayed briefly, then disappears

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Windows XP Pro SP3. I am logged on to the machine as an Administrator. The problems I describe below were not problems until recently. I have installed Firefox and Thunderbird recently, but have made no other changes I can think of. Firefox is now my default browser, but I did not use Thunderbird, and just now uninstalled it.

The command prompt, including ipconfig, cannot be made to appear, or appears for at most a couple of seconds. Any of the following cause the Windows task bar to blank out for a few seconds, as if I had used the Task manager to shut down the explorer.exe process, except that the task bar returns:
Start >> Run >> cmd
Start >> Run >> cmd.exe
Start >> Programs >> Accessories >> Command Prompt
Locating cmd.exe in Windows\System32, and attempting to start it from there

In all cases it is the same whether I use Enter, click the OK button at Start >> Run, use the right click menu, or any other options I can think of.

When I go to Start >> Run and type ipconfig or ipconfig.exe and press Enter or click OK, I see a command prompt window for a few seconds at most (usually just a fraction of a second). Same thing when I try to run ipconfig.exe directly from System32.

For both cmd.exe and ipconfig.exe I have tried this syntax in the Run window:
%SystemRoot%\System32\cmd.exe

I have searched for quite a while, but am now out of options.

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