KB 952265 - Data corruption may occur on a computer that has Forefront Client Security installed

This sounds a lot like the Home Server bug that was finally fixed in the Power Pack 1 release.

SYMPTOMS
Data corruption may occur on a computer that has Microsoft Forefront Client Security (FCS) installed. When this data corruption occurs, you may experience the following symptoms.

CAUSE
This problem occurs because of a known issue of cache coherency between mapped I/O requests and non-cached I/O requests. Forefront Client Security real-time protection uses memory mapped I/O requests for scanning files. This problem affects non-cached I/O requests. It may cause data corruption or cause truncation operations to be unsuccessful.

Special note to Server 2008 core installations: You need to install this manually - read more about it here.