Have a read through this:-
Depending on your BIOS design, you may have a setting that pauses the boot at the CD drive and uses the F1 key to continue - you need to look through the settings to see if this is relevant.
My own preference for resetting the BIOS is to pop out the mainboard battery for 20 minutes which will wipe the CMOS memory.
If this issue has only arisen since you did the upgrade, then it seems extremely likely that it is related.
By the way, have you installed the appropiate XP drivers?
Again a personal preference and not everyone's choice, but rather than install over the top of ME to upgrade, I would always opt for re-setting the drive to zero (using the disk manufacturer's free utility software) and install XP on a clean base and then install the XP drivers and windows updates. As long as you have your original ME CD and key to hand this should be straightforward.
Re-setting the drive does take a little longer but you are assured that all the previous junk is cleared off the drive (but also all your data) and your new o/s system should run at its optimum level of performance - the chances of having to mess around for hours with annoying issues like that f1 problem are also cut to a minimum.
To my way of thinking if you are going to spend time and money building something up, then it makes sense to get the best foundation that you can.
Just a thought.