Please help on which is the correct way to fetch the availability report. Is it with Windows Computer class or with Health Service Watcher class?

Also, please explain the difference between the two classes: Windows Computer class & Health Service W

It depends what you want to achieve. You can set in the top right of the report what you want to count as downtime and you can select any object to run the availability report against.

If you select windows computer then the computer may have critical health state because of cpu \ memory \ disk monitors but this will translate as "downtime" for the server which isn't usually what people want.

 So, in general, I would suggest health service watcher. This looks at the agent and if that is down then it has critical health. It isn't perfect but is probably the best option.

The availability monitors can work well with synthetic transactions as well. So URL availability, database availability but from a remote watcher.

Regards

Graham