NTLDR is the boot loader. It's absolutely needed to boot from any NTFS formated drive. It's a CORE Windows component.
It's NOT offered as a freestanding file.
You probably need to reinstall/repair the OS, or update to 7, to fix that problem. You may try to boot from the install DVD and chose the REPAIR or UPDATE option when offered.
I suggest that you go for Windows 7 instead of Vista. It's faster, use LESS resources than Vista, have many bug fixed,... The installation from Vista is SAFE for your data.