>I always hear to be careful when doing anything with the registry.

Though it is possible to render your machine unbootable by fiddling with the registry, it's unlikely. That being said, I must add the disclaimer that changing the register can render your machine unbootable. More than likely, your app just will not work anymore, meaning you have to uninstall and reinstall.

>What do I need be careful with when adding deleting from VFP6?

You mean deleting from the VFP6 registry hive, or deleting keys/hives using VFP6?

>Any article that can be helpful and is there different concern when working on 98/95 as opposed to NT/2000

That depends on what you are changing in the registry. In summary, any differences between the two OSs are things you do not want to be changing anyway.

>is the registry.prg that come with VFP a solid tools (no bugs)

I can guarantee you that registry.prg has bugs, I just have not found them yet. :-) (Sorry, it's the tester coming out in me.) It is a pretty simple operation, therefore not a lot of complexity to introduce bugs. If you trash your registry, it is more likely to be your fault than the fault of registry.prg.