Service Pack 4 added a disk access every 5 minutes to update
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Reliability\LastAliveStamp, so that an approximate system shutdown time could be posted to the event log in the case of failure.
If you have Windows NT Workstation on a laptop, this feature interfers with power management.
The feature was intended for Windows NT Server only, but it was accidentally enabled for workstation also.
The problem was corrected in Service Pack 5.
If you want to disable this feature on a laptop running Windows NT 4.0, SP5 server, use Regedt32 to navigate to:
On the Edit menu, Add Value name TimeStampInterval, as a type REG_DWORD, and set the data value to 0.