My main Windows XP PC has started exhibiting various signs of slow down.  For example:

• Windows Explorer takes ages to load. i.e. 10-15 seconds, instead of virtually instantly
• Web links clicked from Thunderbird take an equally long time to appear in Firefox
• Word documents load slowly when clicked in Exporer, but instantly when loaded directly from Word

This last point is a clear clue that my Windows Direct Data Exchange (DDE) has become messed up somehow.

To fix it, I ran the command sfc /scannow from the command prompt.  When you do this you'll need your original WinXP CD, as it will check and fix many important system DLLs

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