This is something I had to do this week on a Windows 2003 and an XP machine during the course of troubleshooting an issue.
I found all the steps I needed in these links:
Aaron Stebner has a post on repairing the .NET 1.1 Framework in Windows 2003.
Microsoft article 824643: How to troubleshoot Microsoft .NET Framework 1.1 installation issues (not for Windows 2003).
Microsoft article 830646: How to troubleshoot Microsoft .NET Framework 1.1 installation issues on Microsoft Windows Server 2003.
If these links help one poor .NET troubled soul, all the typing will have been worth it :-).