To address your question about the XP MBR I copied and pasted the quote below. It is correct and says it as good or better than I could. This isn't exactly true with Linux but it is still easier to reinstall Grub than to jump through the hoops necessary for Linux too.
"An MBR is specific to the drive it is written on as it contains info about that specific drive: heads sectors etc. It also holds the volume label for that specific drive as well which again isn't interchangeable. If you copy an MBR from one drive to another you could render that drive inoperable."
You can use the "fixmbr" utility on the XP disk to reinstall the boot load pointer for XP on the new drive.
As far as your ISOs are concerned, it depends on how you made them. If you tried a copy & paste from a running system you will have an unworkable mess.... sorry, but it is better to know now.
I know there are commercial Windows programs to do this but I am not familiar with them. If I were trying to keep an XP install, I would use a live Linux cd and an external drive. Use the rsync command and make a mirror image of the XP install on the external drive. If it uses a recovery partition you should do that one too. Then when you install the new drive restore it back to the new drive from the external drive. An example is below. sda1 would be your XP install. sdb1 would be the external drive. Make sure these partitions are mounted when you run this.
sudo rsync -arE --progress --delete --timeout=120 /media/sda1/ /media/sdb1
To restore it back to the new drive, use Gparted live cd iso,(url below), to partition and format it to NTFS. then run the below on the live Linux cd:
sudo rsync -arE --progress --delete --timeout=120 /media/sdb1/ /media/sda1http://sourceforge.net/project/showfile ... _id=271779
You may still have to coax it back into working correctly but at least you have a chance. I have managed it before. It sometimes goes as smooth as silk, other times there are a few issues to resolve. The closer the drives are, spec wise, the less problems you can expect.
Hope this helps.
Edit: You can skip the middle man, so-to-speak, and go ahead and partition and format the new drive and transfer each partition only once, following the above pattern for each partition. Make sure you keep the same partition arrangement on the new drive or you will have lots of problems.