This sort of thing can happen if you have two hard disks with OS's on them. You need to be careful here because you could inadvertantly take action which could leave you with a U/S boot drive.
My first thought is your best chance of success is using the recovery console, for which you will need the Windows XP CD. (In my opinion you should ALWAYS have an operating system CD because there is so much you can't do without it.)
Few things you could check :
1) Is the 'new' drive set as primary master with the 'old' drive on one of the other channels ? If yes, try disconnecting the 'old' drive completely and just boot with the new one.
2) Check the boot sequence in BIOS. Sometimes this works.
3) Make sure there are no floppy disks in the drive.
4) Is your primary HDD formatted as FAT32 or NTFS ?
However I come back to my first point, I suggest you do nothing significant until you have the CD.