hp a530n start up problem

Newbie on this board so forgive me if this has been covered, but I cannot figure out a way to describe the problem to find it on a previous thread.

I am working on an HP a 530n desktop. It was booting properly before I got my hands on it.. but, as per the customers wishes, I created and installed the recovery disks. XPSP1

Did all the Windows Updates and the HP updates before I installed SP2 from disk (a very often used disk, not an untried one).

Now, on alternating boots, the computer will stop responding BEFORE it gets to the bios screen or anything else. The yellow HD activity light stays on (solid) and the power light is on.

When I force off the power and reboot, it boots normally. No error codes, "your computer has recovered..." no indication that there was ever a problem.

This did not happen with SP1 or before I reinstalled the system.

I have removed, replaced and reinstalled all the hardware, CMOS battery, PSU but all to no avail.

Ideas, PLEASE.