Chkdsk

Volumes that have file system errors are known as dirty volumes. If a volume is dirty, it has experienced file system errors, and you must run Chkdsk on the volume to repair the problem.

Every time you start a computer running one of the operating systems in Windows XP or the Windows Server 2003 family, Autochk.exe is called by the Kernel to scan all volumes to check if the volume dirty bit is set. If the dirty bit is set, autochk performs an immediate chkdsk /f on that volume. Chkdsk /f verifies file system integrity and attempts to fix any problems with the volume.

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/defrag.mspx?mfr=true