First determine which versions of .NET Framework are installed and
whether any service packs have been applied:
See:
http://support.microsoft.com/kb/318785
Next download to your hard drive each version of NET
you have on your PC. download the installation files
(do not use Windows Update) from this list
"Where to download the .Net Redistributables"
http://www.pagestart.com/netframeworkdwnldlinks.html
Then uninstall each version .NET you have on your PC,
Starting with the highest version first and finally uninstalling NET 1.x
(Remove 1.1 hotfix after removing .Net 1.1) if its on your PC.
Now use the .Net Cleanup Tool to
remove the last vestiges of .Net
..NET Framework Cleanup Tool:
http://blogs.msdn.com/astebner/pages/8904493.aspx
"This .NET Framework cleanup tool is designed to automatically perform a set
of steps to remove selected versions of the .NET Framework from a computer,
It will remove files, directories, registry keys and values and Windows
Installer product registration information for the .NET Framework.
The tool is intended primarily to return your system to a known
(relatively clean) state in case you are encountering .NET Framework
installation, uninstallation, repair or patching errors so that you can try
to install
again". I used this tool after using the Add or Remove Programs uninstall
feature. After using the Cleanup tool only a few folders and files still
remained, so I deleted these manually (See below)
..Net folders - After deleting all .Net Redistributables:
C:\Program Files\Reference Assemblies\
(Delete the Reference Assemblies folder and all subfolders )
C:\WINDOWS\assembly
(Delete the assembly folder and all subfolders)
C:\WINDOWS\Microsoft.NET\Framework
(Delete the Microsoft .Net folder and all subfolders)
C:\Program Files\MSBuild\Microsoft\Windows Workflow Foundation\
(May or May not exist)
Next reinstall each version you had on your computer
working your way back up to the last (highest version #) of NET.
NET Framework Setup Verification Tool:
http://blogs.msdn.com/astebner/pages/8999004.aspx
"This .NET Framework setup verification tool is designed to automatically
perform a set of steps to verify the installation state of one or more
versions of the .NET Framework on a computer."
Then check for any .net security updates/hotfixes using Windows Update.