I own a domain that I have had for over 18 months.

I was just recently sent a "cease and desist" letter claiming I have
infringed on a US trademark from an Internet company.

To protect all parties - This example will be fictitious.

Ex. The company name is etrade.com.  They sent me faxes stating they
trademarked that name plus etrade and etradestocks

My domain is e-tradecentral.com.  I am an affiliate site focusing on
financial services, but have nothing to do with online stock trading. 
I do not state anywhere on my site that I am affiliated with their
site.

My Question:
-Do I have a good chance to keep the name (I have spent a good amount
of time increasing the rank via SEO work)?
-If I change the content of my site to state EC (rather than
Etradecentral)- would that be sufficient?
-What other remedies are possible if neither of the above work?
-What advice would you give on how to handle this matter?

Thank you,

First of all, you need legal advice, which cannot be provided through
something like Google Answers. You really need to consult a lawyer
with experience in this area. You might try your local bar
association, or possibly the Electronic Frontier Foundation
(www.eff.org) for leads, though I think this is a little outside of
what EFF normally deals with.

Secondly, as a purely non-legal outside observer, I will say that
domain name disputes like these seem to be quite common, and a common
settlement seems to be that you add a disclaimer to your home page
saying you are not affiliated with the other site, plus a convenience
link redirecting those who came to your site in error.

This is a very fact-sensitive inquiry.  Did you register your domain
name before or after the other company began using their name as a
trademark?  If after, why did you appropriate a domain name that is
similar to someone else's trademark?  If before, you should have
better rights to the domain unless you simply registered it and were
not actually using it as a trademark.  Trademark law says, generally,
that "first in time is first in right."  Mere registration of a domain
name is not sufficient to accrue use in the name as a trademark for
purposes of winning a fight over the domain--you have to actually use
the domain name as a mark to build up rights.

Here are some examples:

Assume I registered bloxnall.com on October 1, 2003.  Then, on March
15, 2004, I began using the word "bloxnall" as a trademark for my
particular brand of golf clubs.  I then get a letter from the owners
of the trademark "bloxnell," who have been using that as a trademark
for their brand of golf shoes since April 1, 2004.  You win, since you
have priority of TM use (March 15 versus April 1).  But if the owners
of the bloxnell mark show actual trademark use on December 1, 2003,
then you lose because mere registration of a domain is not the same as
trademark use (December 1 versus March 15). See the EPIX case at
http://caselaw.lp.findlaw.com/data2/circs/9th/0135144p.pdf

So--when did you start using the domain as a trademark and was it
before or after the company who sent you the cease and desist began
using their mark?  If after, then your best hope is to make an
argument under traditional trademark law that your use is not
infringing.  This assumes you are actually using the domain as a
trademark.  For example, if I register bloxnall.com but all I ever do
with it is use the site to sell golf carts and I never sell
?Bloxnall-brand? golf carts, I have not used bloxnall.com as a
trademark for golf carts and I have no rights to stop someone else
from selling Bloxnall-brand golf carts?I have no TM rights in the
name.

Yes you are intitiled to keeping your domain name. Why? Because it
seems that you have a contract with this domain name company. What
they are doing is called, breech in contract, you could sue. Even if
you dont have a long term contract, the domain is yours.

Getting a Domain name and Registering a Domain name are different.

Anyone can buy a Domain name.  But, is it registered.  If you
registered your domain when you purchased, then you can keep it.  If
they find that the domain name is not registered then it is fair game.

Sorry, I cannot allow actkid's and kielinen's comments to go without
response.  In short, they are both wrong.  Assuming that they both are
continuing to follow this thread, let me ask them this hypothetical. 
Assume that someone at Nike responsible for renewing domain names
forgets to renew nike.com, and I manage to register it.  Moreover, I
begin to use it to sell Reebok athletic shoes.  Their comments suggest
that I would be allowed to keep that domain simply because I "have a
contract" with the registrar and because I "registered" the domain
name.  That is absurd.  Trademark law says you cannot use a domain
name that incorporates someone else's trademark (even if they screwed
up and forgot to renew it) to sell related or competing goods or
services.  You only have rights to a domain that incorporates someone
else's trademark if (a) you registered the domain name first AND (2)
you were using the domain as a trademark first.