We have a free web  part that displays a welcome message to the logged in
user. Its called "Hello Web Part". It may save you a little work.
Download it from our online store.

after installing this, I got the following
error:

Server Error in '/_layouts' Application.
--------------------------------------------------------------------------------

Runtime Error
Description: An application error occurred on the
server. The current custom error settings for this
application prevent the details of the application
error from being viewed.

Details: To enable the details of this specific error
message to be viewable on the local server machine,
please create a <customErrors> tag within a
"web.config" configuration file located in the root
directory of the current web  application. This
<customErrors> tag should then have its "mode"
attribute set to "RemoteOnly". To enable the details
to be viewable on remote machines, please set "mode"
to "Off".


<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="RemoteOnly"/>
</system.web>
</configuration>


Notes: The current error page you are seeing can be
replaced by a custom error page by modifying the
"defaultRedirect" attribute of the application's
<customErrors> configuration tag to point to a custom
error page URL.


<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="On"
defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>

Thanks, Bamboo sulotion helpped me to fix this
problem.Works great now.