It`s some program or piece of hardware that is not complying with Windows power management.What program or piece of hardware did you install recently ? Uninstall it.

Try a 'System Restore' to a date before the problem started.
http://blogs.msdn.com/b/microsoft-help/archive/2010/02/21/system-restore-in-windows-xp-vista-and-7.aspx

Go into 'msconfig' startup and uncheck everything except firewall (if there).
http://netsquirrel.com/msconfig/index.html

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