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).
Windows Reinstallation Guide and Related Wikies See here for other wikies such as Customising and Using Windows 8.1, Dell Wireless Cards and Unofficial Drivers.