Depending upon the age of the drive, and how "mission critical" the data is, repurposing a drive into a new build is ok. I recently upgraded my main rig and used the old 500GB drive, and it is still going strong. The drive was about 3 years old.
If the drive is 6+ years old, and the computer is used a lot, you might want to consider replacing it. Also - older hard drives can be IDE, where the new computers are all using SATA. There could be big speed differences between a SATA I and SATA III drive as well.