>2. Where to use LockScreen? Because sometime when I click on any object the form slightly shake.

You would use LockScreen when there're many changes to the objects on a form and you want them to display at once to avoid flickering.

>3. Is it safe to use SP2 now? As I am not fully aware of it.

You will have to install the latest SP2 hotfix as well http://www.berezniker.com/content/pages/visual-foxpro/vfp-90-versions. Keep in mind that not all bugs introduced in SP2 have been eliminated by hotfixes http://fox.wikis.com/wc.dll?Wiki~VFP9SP2BugList. You have to see if they affect your application or not.

>4. In case of SP2 is there any major changes in project?

There have been a lot of changes to Object-assisted Reporting.