Tedster my boy, I just successfully rebuilt 2 eMachines (new Power supply motherboard, processor, memory and CPU) and used the old eMachines C drive as is...
Boot up on the old drive, wait for Windows to load. This may take a few "cold boots" and black screen stalls. After 4 or 5 minutes both systems finally booted to the Windows log-on screen telling me that I had to Activate Windows before I could log-on. I said yes to activate. The Activation screen came up and I selected to call Microsoft.
A new number sequence key was generated. I called the automated Microsoft "person" and gave "her" the key sequence. She told me that it was not valid. A live person came on the line and told me to "change the key"... There were numbers in the key boxes already. I was told to enter the key numbers on the label stuck to the eMachines case, and hang up the phone, and restart the Activation process. I called the automated Microsoft "lady", gave her the new sequence numbers and this time the activation was successful.
After loging into an Activated Windows, I installed the new motherboard drivers and the MS Updates. I ran registry cleaners, defragged the hard drive and installed antivirus software. All the customers settings, files, folders and programs were intact and operational. The systems run fast and stable