Acer aspire 4720z fails to start desktop

Hello,



my acer aspire 4720z doesn't boot to desktop then shuts down instead.this problem started with window 7 so i could use only linux ubuntu 9.04 but when i tried to ovewrite the Linux GRUB by applying 'bootrec.exe/fixmbr.the GRUB was lost yet when windows7 booted it couldnt start up to the desktop enviroment and immediately after few seconds it shuts down,pliz help me!