What's Wrong With My Computer?

In the morning when I get up I switch my computer on at the wall and then press the button on the tower to start it up.
However the last few mornings after I have switched it on at the wall it won't start up when I press the button on the tower. Instead there is a green light on the back of the tower that flashes for about 10-15 mins and it won't let me start it up till that stops flashing and is solid green.
This morning it took nearly 40 mins before the green light stopped flashing and I could start it up. Once its going its fine.
Any ideas whats going on here please and what the problem is?
I know i could leave it switched on at the wall at night but I like to turn all my plug sockets off at night otherwise i don't feel safe going to bed and sleeping.

When a computer starts up, it does a POST. (Power On Self Test.) Generally, you can get diagnostic information from the pattern of flashing lights on either the front or back panel to tell what the POST is doing.

Or, to put it simply, it sounds like you might have a hardware problem. Either do a bit of googling for your specific make/model or take it to a reputable repair shop.

sounds like you just need a Formatt......

Nope. From the symptoms, I don't think it's even getting as far as trying to boot from the hard drive.

This looks like a useful guide.

To see if it is the disk/operating system, you'd need to boot from a Linux live cd to see if it boots quickly (Which would indicate a hard drive and/or corrupt operating system problem.) or still takes ages. The latter would indicate other hardware issues. Most likely memory or video card maybe...