Hi:
According with the description that you gave us, looks like a problem with
the HDD. When you make a cold startup, the HDD needs to start to spin up.
The BIOS is ready, but the HDD is not ready yet, thus the BIOS is not
detection the HDD.

When you make a hot reset the HDD is spinning and the BIOS do not have
problems detecting  it.

Try to call HP, and explain them the porblem. May be just a BIOs update will
fix the probelm.

In addition to Juan's suggestion, you can download a HDD diagnostic from
the hard drive manufacturer's web site. For example, if the drive was
made by Seagate or Maxtor, you'd go to seagate.com and look for a download
there. The diagnostic may warn you about a disk which is about to fail.
In which case, at the very least, you should back up your data to another
hard drive, so you don't lose everything. Check the label on the drive,
or use a utility, to determine who makes the drive, so you can find a
diagnostic. The diagnostic exists, to justify warranty returns to the
manufacturer, so the diagnostic won't necessarily give a long explanation
of what is broken.

Hard drive manufacturers also have transfer software, for when you
purchase a new internal hard drive. The transfer software can be used to
transfer the contents of the old hard drive, to a new hard drive.

Now is also a good time, to think about your backup strategy, such as
what would happen if the hard drive simply stopped working. Is the
user data on the disk, backed up somewhere ? You can get a USB
based external hard drive, to do some backups.

The symptoms make it sound like the hard drive is slow to start up,
and that may be why the BIOS has concluded you have no boot device.

I learned my lesson once, about how quickly you should respond to an
ailing computer. One night, I started getting the "click of death"
from a hard drive. I checked the drive and I could still see all the
contents. But I was tired, and decided I'd do a backup the next day.
The next day, I turned on the computer, and "no hard drive". She was
dead and gone. And with it, all my data :-(

So don't leave it too long, to do something about it.

   Paul