Is it possible that a laptop or a computer isn't compatible with windows 7 even if it has 2Gb of memory and a decent 2Ghz intel processor and the hard drive has 200Gb free space?
It is not the hardware specs that are causing the problem. Just because a computer/laptop meets certain hardware requirements does not mean it will support an operating system.
What are the reasons for this incompatibility?Specifically incompatible in WLAN Card. What can I do to make it compatible.?
Drivers are what affect compatibility. You will need to make sure you have Windows 7 drivers for all the components that require drivers. Windows 7 has very good driver support but you may need to obtain Windows 7 drivers directly from the PC maker (Dell in this case) or from manufacturers of the internal components. As @Sim has also pointed out, you may be able to use Vista drivers if you cannot find Windows 7 drivers.
Also, depending on whether you're using a 32 bit (x86) or 64 bit (x64) version of Windows 7 you will need to find 32 bit or 64 bit drivers.