Windows XP Home Addition ver 5.1 (which includes Service Pack 3)
This issue is addressed at:
http://support.microsoft.com/kb/310601/
and is supposed to be fixed by the latest Service Pack. This issue still is not fixed with SP 3 or has been "unfixed" with an update. Using the "Power Options Properties" with "System standby" set at 45 minutes the computer will never go into
standby. With 30 minutes for "System standby" the power down feature to standby works properly.