Hi,
1. Which tool are you trying to install on the computer?
2. What is the error message you receive when you try to install the tool?
**Method 1:**Start the computer in clean boot and then try to install the tool to check if any third party program is causing the issue. To configure your computer to clean boot refer the following link.
http://support.microsoft.com/kb/929135
Note: After you have finished troubleshooting, follow step 7 in the article to boot to normal startup.
**Method 2:**Repair the .NET Framework
To repair the .NET Framework you must uninstall multiple versions of the .NET Framework by using the .NET Framework Cleanup Tool, and then reinstall the components.
To do this, follow these steps:
a) Download the .NET Framework Cleanup Tool from the following link:
.NET Framework Cleanup Tool User's Guide
http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
b) Using the .NET Framework Cleanup Tool, uninstall the .NET Framework 1.0, 1.1, 2.0, 3.0, and 3.5.
Note: The cleanup tool does not let you remove the .NET Framework 2.0 in Windows Vista or later versions because the .NET Framework is installed as an operating system component.
c) Restart the computer.
d) Download and install the following components:
.Net Framework 1.1:
Microsoft .NET Framework Version 1.1 Redistributable Package
.Net Framework 1.1 SP1:
Microsoft .NET Framework 1.1 Service Pack 1
.NET Framework 3.5 SP1 (this will also install .Net Framework 2.0 SP2 and .Net Framework 3.0 SP2)
Microsoft .NET Framework 3.5 Service Pack 1
e) Restart the computer.
f) Visit Windows Update, and then check for and install updates.