Was this an in place or side by side upgrade? In Place upgrades are somewhat tedious and I have done 100s of SQL server migrations\upgrades with all side by side due to the most safe method as per my experience, reliability, easiness etc that it offers overall.

I think you should check on server Control Panel --> ALL Programs to see if any SQL 2005 component still there? Also, you can check registry files to see if entries are there for SQL 2005. Also, you can check services.msc, config manager etc to confirm various things for this.

Thanks,

Vikas