There might be a problem with ur disc drive, or the installing disc, or with ur boot device.
Try installing Win 7 from a 4GB or greater Usb drive
Detailed steps:
Format the usb drive in NTFS format.
open command prompt, start-programs-accessories-cmd.
Type the following commands without quotes and press enter.
"diskpart"
"list disk"
Now note the disk no of the usb key, disk 1 or disk 2. Be careful while selecting the disk. Use size as reference.
type the following in sequence
select disk 1 (where disk 1 is the disk number of usb drive)
active
list partition
select partition 1
active
exit
Now insert you disc, or mount the iso on a virtual drive(using deamon tools)
From cmd, browse to boot directory the real/virtual cd drive.
G:\cd boot (where G: is the drive letter of the virtual drive)
Now, when u are here, type this
g:\cd boot\> bootsect\nt60 h:
where h: is the drive letter of the usb drive.
Now exit cmd.
copy all files from the mounted image to the usb drive.
Then boot from the usb drive.
If this works, then ur disc drive is creating a problem.
If this does not work, download a fresh copy of win 7, and burn it on dvd or mount it and install using usb drive.
If both these methods do not work for you, then there is a hardware problem,(boot device, or something else), which is beyond my expertise of knowledge.