Kinda weird standby problem

Started doing it a couple of weeks ago .

Havent been into the settings or options for months .

XP home with all the latest security upgrades and patches .

USB broadband modem .

Sometimes when i click on disconect - theres a audible click from the pc
and the monitor goes black .

The fan still whirrs and clicking a mouse button or pressing a key on
the keyboard has no effect .

Monitor still seems to be getting a signal as the monitor only switches
itself off after i press and hold in the pc on/off button for 5 seconds
or whatever to switch the pc off manually .

Like i said it started a couple of weeks ago and seems to be a random
event as it can go a week e.g. without doing it .

ctrl-alt-del has no effect and calling it a 'standby' problem seems to
be the best description to give it .

.