1. Well, if you're still in the operating system looking at the recycle bin; you'd open up the recycle bin and double-click on "NTDetect.com" to restore the file to its previous location.
If you're unable to boot the operating system, it's going to be a little harder... You'd need access to a copy of "NTDetect.com" and a means to copy it back to the hard drive.
2. Operating System. What version and service pack level were you operating with? For example, Windows XP Home Edition SP3?
3. What Windows XP recovery/restore or installation CDs do you have?
4. Options:
a. Boot Floppy. If you have a floppy drive and access to a working Windows XP system, you could build a boot floppy containing the necessary boot files: NTLDR, NTDETECT.COM, and BOOT.INI. You'd need to modify the boot.ini file to point to the location of the operating system on your hard drive. Trick is to format the floppy on a Windows XP machine so its boot record is written to look for NTLDR as the bootloader file.
You'd boot Windows XP from the floppy where the "boot.ini" file is configured to point to the operating system on your hard drive. Once you're back in the operating system, you'd copy the NTDETECT.COM from the floppy to the active primary partition of the hard drive (normally C:).
b. Recovery Console. If you have the appropriate Windows XP Install CD, you can boot to Recovery Console from the CD and use its limited command capability to copy NTDETECT.COM back to your hard drive (from CD, floppy, USB flash drive).
If you don't have Install CD, you can download a recovery console iso image off Internet, write (not copy) the iso image to CD, and then boot that CD. You'll still need access to media containing NTDETECT.COM file however.
c. Slave Hard Drive. Pull the hard drive out of your system and "slave" it to a working Windows XP system. Copy the NTDETECT.COM from their system to yours. Not sure whether you need to match versions and service pack levels here. (I'd probably make sure NTLDR and NTDETECT.COM have same "modified" date to ensure they're consistent. Mine say "4/14/2008".)
d. Boot to Rescue CD. Boot to a Live CD containing an operating system that supports "writes" to NTFS file system and use it to copy "NTDETECT.COM" file from whatever media you have it on (floppy, CD, USB flash drive). Many different Live CDs to choose from. Ubuntu 9.04 Live CD will work.
5. Let us know which way you want to go...
