I am planning to buy any Dell core i5 laptop costing around $600 but I don't want Windows 8 rather want to have windows 7. Is there any option within same range that I can get windows 7 pre-
No. You will need to 1) spend more for a Latitude, Vostro, or XPS, 2) check Dell's outlet store (or local stores like Walmart or Best Buy), or 3) purchase your own Windows 7 license.
Or 4) purchase a machine with Windows 8 Professional and not Windows 8, this will give you downgrade rights to Windows 7 Professional but you will need to contact Dell for the Windows 7 Professional with Service Pack 1 64 bit Reinstallation DVD. This option is limited for Home systems so this might be an equivalent to 1).