Windows 7: my windows seven is not starting up

i have a problem with its starting,it has been installed properly and works perfect when there is windows cd in its cd rom,otherwise it does not start and says operating system not found. Please tell me what to do.

only starting fine with windows cd in cd/dvd drive... hmm strange. Try to start with dvd in drive. Does it say something like "press any key to boot from dvd".? If you press any key it should start from dvd and Windows 7 setup appears. But you don't press any key, isn't it?

check the boot device priority in bios. Is it floppy, cd, harddisk? tell me what it is now

Boot you pc using Windows 7 installation DVD and select "repair you computer" then enter "command prompt"

DISKPART
LIST DISK (how many disks are listed?)
SEL DISK 0 (how many partitions?)
SEL PART 0
DET PART
SEL PART 1
DET PART
SEL PART 2
DET PART
exit

Which partition is active? only one?

What is the error message when starting without dvd in dvd drive?