Here is the fine print regarding the Free Windows 10 Upgrade from Microsoft.
This is purely opinion on my part:

- Any computer that was running Win XP originally would be a poor candidate for this upgrade. A computer of that era will not have enough memory or CPU speed.
- Any computer running Vista originally, see above statement.
- I would wait 3-6 months after the upgrade is made available before I would attempt the upgrade. I think a much more stable and available release should be available in that time frame.