When you reinstalled Vista, it's bootloader
should have seen XP and automatically dual booted.
It sounds as if the reinstall has changed which partition is now the system active boot.
You need to identify which partition is now being booted from (admin tools/comp mgmt/disk mgmt)
and make sure
all the necessary boot files are on
that drive.
Read the Wiki troubleshooter, and it'll tell you everything you need to copy.
http://neosmart.net/wiki/display/EBC...ing+Windows+XP