The most common cause of this is antivirus software. I've heard reports of Exchange-aware antivirus software being out-of-date and needing updates. As a quick test, you can temporarily disable your antivirus software and see if the store settles down. If that doesn't resolve it, or if you aren't using antivirus software (why aren't you? [g]), then have a look at Troubleshooting high CPU utilization by store.exe.
Dig Deeper on Microsoft Exchange Server Storage Management
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