go to start-->control panel-->add/remove programs then in the list look for .net framework 1.1 and choose change/remove and then uninstall.
This will remove it although not fully from your system. To get rid of it completly you would need to alter the registry which can be dangerous if you dont know what your doing.
To reinstall go to microsoft update choose custom and then click on one of the update types on the left and scroll through them till you see .net framework 1.1.
If you want to try and repair .net framework try this:
Repair the .NET Framework
The following are three ways to repair the .NET Framework.
Repair the .NET Framework 1.1 by using the Windows Installer cache file
To repair the .NET Framework 1.1 by using the Windows Installer cache file, follow these steps:
1. Click Start, and then click Run.
2. In the Open text box, type Installer, and then click OK.
3. On the View menu in Windows Installer, click Details.
4. Do one of the following, depending on your operating system:
In Windows XP, right-click the Name column header, and then click Comments.
In Windows 2000, right-click the Name column header, and then click Comment.
5. Right-click the cached Windows Installer file that has the Microsoft .NET Framework 1.1 RTL x86 enu comment, and then click Repair.
6. After the repair process has completed, quit the Installer.
To fully remove .net framework microsoft suggests this Although Id try just the basic unistall method first that