Another NTLDR is missing issue- Harware Upgrades

I was upgrading a system for a co-workers kid. He was using a Compaq 500Mhz with 64MB of RAM running Windows XP. I was suprised the thing even booted up. Nevertheless, I built him a 1.9GHz AMD Athlon, with a Biostar motherboard, 256MB DDR400, and a 128MB video card. The hard drive is a Maxtor 20GB. When I brought the system up, I got the infamous NTLDR is missing. Is this a case of doing the fix /mbr? His OS was intact and running on the old Compaq.