Pragati,
This is a question that has been answered several times in several places.
http://www.google.com/search?hl=en&q=.net+remoting+vs.+dcom+vs.+web+services
You will find many answers.
Top 3 differences:
1. Web Services are an open technology-agnostic standard. Remoting is .NET
specific. DCOM is Windows DNA specific.
2. Load balancing DCOM components is relatively more complex than load
balancing Web Services
3. Web Services work through HTTP - so it is easier to traverse firewalls
compared to Remoting or DCOM.
Regards,
Pandurang

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blog: www.thinkingMS.com/pandurang
> while using DCOM where exactly we need to set the component setting ?In short
> how we can call remote COM component in existing asp or vb application ?
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> decide when to use the above mentioned technology from performance point of
> view ?