Blank screen , I think changed to xp from windows 7

Hello,
My pc has been changed from windows 7 to windows xp while doing something with the compatibility settings for an application.
Now when I switch on my system it starts, but a blank screen appears with a cursor without any wallpaper or icons.
Nothing even happens when I use touch pad but moving of the cursor.
Any things happens that is only through task manager.
Please help me!
I hav no clue of what needs to be done.