Have you done any modifications to ExplorerFrame.dll, such as skinning with 3rd party applications etc. ?
Have you tried logging in with a different user account, do the crashes occur as often?
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High level of hard disk fragmentation sometimes causes windows explorer crash. Defragment your disk.
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Suggestions (a list of what to try):
1. Take good hard look at what software you have installed, a good chance some software is causing the error.
2. Perform a clean boot:
Start -> msconfig -> general tab -> selective startup -> clear the load startup items
Start -> msconfig -> general tab -> services tab -> hide all Microsoft services -> then Disable all
Restart
3. Start -> gpedit.msc -> Computer Configuration ->...