Could be due to few things,
1. your hard disk might not be the first boot device from BIOS.
- try and set the HDD as 1st device from the bios menu
2. system POST is Delayed.
-this is a check the bios does to verify your hardware is working well
-if you feel this is the case try to reset the bios to defaults and then make sure to set the HDD as 1st boot device
3. could be signs of a failing hdd

lets not come to that...
4. could be due to a windows fault.. come on after all we are talking about Microsoft right

- try to reinstall your copy before attempting any hardware repairs
hope this helps..
cheers