Think.....
Fans spin, power?
At what point during the boot up process does it fail? Does it get to:
1. CMOS Check Screen.
2. WIndows 'failed to load' screen, giving you a choice to boot normally or start in safe mode
3. The normal WIndows Boot Screen.
Can you get into the Setup (press 'Delete' as you boot or F2, depends on the CMOS it's running). If you can get into that then I would say the Power Supply is okay.
Places to look, things to look for/consider.
Check all connections
Any stuck keys on the keyboard?
The kids.... spilt a drink on it? dunked the mouse in a drink? (dont larf, mine did once)
Swap the keyboard & mouse, try again (the current CMOS settings may have it stall the load process if either of these are faulty)
You say it's clean inside... Please tell me you havent stuck the hose from the hoover in there and had a wave about? Is the hose end metal or plastic?
Check all connections inside the box for tightness and correct location
have the kids 'posted' a note to granny/biscuit etc in the CD slot
Has the dog/cat bleep on it?
Does the SCREEN work? more than one PC has made it to the re-cycle because the screen died and they never sussed it
Good luck...
This posting may have been helpfull, on the other hand probably not... Either way, I may have had drink when posting it in which case it's probably not as helpfull as it could have been
Work safe, and remember all technical problems can be resolved with a hammer, the trick is knowing where to hit it and how big a hammer to use (I often find it may also help to have a spare bit in the shed to replace the bit you have just mullered

)
Work in Progress (or not as the case may be) I give you....
'O-no' the truck.Comments on 'O-no' the truck may be left
hereWork in Progress (or not as the case may be) I give you....
'Truck'Comments on 'Truck' the truck may be left
hereOwning a 2.8 is a hobby in it's self...