May we presume that you attempted removal via control Panel - Add/Remove?
Please indicate what operating system the machine is running. Win 98, Win Me, Win 2k, XP
If Add/Remove has not been effective, you may use the New Dot Net Uninstall Instructions, that are provided at their Website: http://www.newdotnet.com/removal.html
If you cannot access the above linked site or your browser shows the error "this page cannot be displayed" , then you may have MVPS Hosts File in place blocking the site.
The removal tool at the newdotnet site is reliable.
However if the version of newdotnet that your have on your machine if older/prior to July 11, 2001, You may have some additional problems Including loss of Internet connectivity.
WinXp internet connectivity can be restored using
WinsockXPFix
Earlier versions of Windows will require LSPfix to restore internet connectivity. http://www.cexx.org/lspfix.htm
If you are not familiar with these tools to restore internet connectivity you should post a HJT Log in the HJT Forum and wait for instructions from a trusted HJT advisor.