I'd usually start with my suite of cleanup tools, do some manual cleanup if needed, apply all the software and security updates, and then give it a day with some light test usage. Then I'd re-run the tools to see if they picked anything back up. If not, I released it back to the customer. If anything at all came back, I'd backup their data, pull all the product keys I could (Office, Photoshop, etc), nuke the OS, and reinstall what I could as close to the original as possible.