I have a .com domain ,do I need a company registered with the same name , to prevent someone from claming it

No...

You would only have a concern if you took another company's domain. There could be an issue with intellectual property rights.

For example, if IBM did not have a website yet and you took www.ibm.com, then they might have legal recourse.

If you own a domain, then don't forget to set it up with automatic renewal though. If you forget just once, it's a good way to lose your domain name - someone else could buy it.

No. Some people make a VERY good living camping domain names. As long as your have some justification for using that domain name, you'll be fine. For an interesting read check out the story behind www.nissan.com (http://www.nissan.com/Digest/The_Story.p...

It depends upon the legislation of your country. The domain registration is separated from company registration, but in some cases a company can prevent you on using a domain case they establish a trade mark under the exact same name. The only way to bee 100% sure that no company can prevent you from using a com domain is to simply register the name on the company registry. There is no need to actually form a company. In most countries any citizen can register a name on the company registry files for future use.

You can't do business or turn a profit under an established company name, you would have a problem there. But I don't think that you have to worry about loosing your domain name. If it's available you can lease it. After-all you never own the domain name.

I do think the Nissan.com site is a fraud, in my opinion.