Let me try and explain..........................
If when you plug in the power supply and no little light comes on the front - and while plugged in and it will not boot. The you have a higher problem than just a flat battery, everyone here is assuming you have totally discharged your battery and the reason for not booting is purely due to the flat battery.
Hense the lack of response - If you can be more specific like.
1> when plugged in (regardless on if the battery is charged, charging or even in the unit) you should still be able to at least see the dell logo when pressing the power button.
2> When plugged in does the little white light even glow on the front with the battery in.
And an OS is called an operating system like XP, Linux or OSX.