Normally the .NET Framework is uninstalled using START --> CONTROL PANEL --> ADD/REMOVE PROGRAMS.

However, if you have difficulty (read: impossible) un-installing the .NET Framework, here is a not-for-the-faint-of-heart link to Microsoft and a step-by-step article. Good luck....
http://support.microsoft.com/kb/824643/

As another alternative (and simpler) just Google "Windows Install Clean Up" and look for the software to download...