I have a Compaq S5500UK PC with an ASUS mobo which has an Nvidia nForce 2 Chipset (specs in sig). Do you have an Nvidia Chipset, and if you do, have you upgraded your chipset drivers recently?
I experienced a problem similar to the one your are experiencing. After upgrading my Nvidia nForce 2 Chipset Drivers from version 4.27 to version 5.10, I noticed that after rebooting, there was a 30 second freeze time just before the OS loads, and the Windows XP splash screen animates.
This continually ocurred, until I uninstalled the entire suite of nForce Chipset Drivers completely, and rebooted. The system worked fine. It was only when I did process of elimination did I find out that the 'Nvidia IDE Driver' installed from chipset drivers version 5.10 was causing the boot freezup.
To fix this problem, I had to make sure that a version other than the IDE Driver in the chipset package 5.10, was not installed. Currently, I am using the older Nvidia IDE Driver version, from the 4.27 package, but all other drivers are upgraded to the ones in version 5.10.
This may not be the same problem...but at least it is something you can either jump out on your seat on, or just eliminate it