Dell Inspiron 1501 booting issue

[PraiseJeebus]

Hi all, I have an unusual issue with a Dell Inspiron 1501. When you press the power button to turn on there is no Dell BIOS screen, the screen remains black (with the back light on) for approx 20 - 25 mins and then boots perfectly into windows. Any suggestions as I'm stumped.
I have taken the hard drive out and scanned it with hdd Regen it scanned perfectly, have ran manufacturers smart and drive tests. I have changed the RAM to see if that is the issue - same out come.
Thanks in advance
Paul