Summary: .NET framework is an important support library for many programs. Unfortunately, its versioning and updating leave a lot to be desired.
Do i need Microsoft .net framework 1.1 and 2.0 service pack 1 and 3.0 service pack 1 on my desk top computer? I don't develop new software.
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The fact that you don't develop software actually has nothing to do with anything here.
.NET is most likely used by other software you have installed on your machine.
.NET versioning, unfortunately, is apparently a mess.
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To cut to the chase: I wouldn't delete a thing. It possible that you do, in fact, need all those different versions of .NET - or not. We could go through a long exhaustive search to see which programs you have installed might require one version or the other, but in my opinion it's just not worth it.
OK, just what is the .NET framework?
To grossly oversimplify, it's a package of common support software that can be used by programs so that they don't all have to re-write the same software over and over again. More specifically, it's aimed at users of Microsoft's newer versions of programming languages like C#, Visual Basic .NET and others. The .NET framework provides a broad array of functionality that can be used by programs written in those languages to perform common tasks, most commonly things that involve interacting with Windows itself.
So you may well have programs installed on your machine that rely on the .NET framework, and thus deleting .NET will cause that program to fail. Over time, more and more programs have come to rely on .NET, so chances are actually pretty high that you have at least one and perhaps more that rely on .NET.
You get the .NET framework either of three ways:
It might already be on your machine. Certain versions of the .NET framework are pre-installed on some versions of Windows.
A program you install might install it. The .NET framework is available in what are called "redistributable packages", which means nothing more than software vendors can include it in the software they deliver to you. If a program requires the .NET framework, then it may include it on its CD or in its download, and install it automatically as part of installing the program.
You may be instructed to download it. The .NET framework tends to be large, and because it's also fairly common these days, many vendors are opting not to actually include it in their distributions, but instead ask you to download and install it if you don't already have the required version.
Now, about those versions.

The .NET framework versions installed on my machine
Normally, it's safe to assume that version 2 of some software replaces version 1, and that version 3 replaces both 1 and 2. Each later version includes everything in the prior versions, so that the prior versions are no longer necessary.
Apparently the .NET framework isn't "normal". For example, software that requires .NET framework version 1 might not work if only version 2 is installed. In fact, you'll already note that while most software upgrades replace the previous versions, .NET framework installations do not. If you install version N+1, version N remains.
That's kind of frustrating.
But the solution is simple: leave them there. Yes, they eat up more disk space, but unless you really know what you're doing or want to spend a lot of time experimenting (back up first!), just leave them there and get on with your life.
Rumors are that the mess will get cleaned up in a future version, but in all honesty, I'm skeptical.
Related:
Is it safe to uninstall updates? Some updates might be safe to uninstall, but doing so makes some risky assumptions. Most programs keep only the patches they need.
Are automatic updates a good thing? Unless you're willing to pay a lot of attention on a very regular basis, automatic updates are an important part of keeping your machine safe.
Article C3563 - November 13, 2008
what was the response to the question about the continual attempt to automatically install .NET Framework 1.1 SP1 Security Update after it has all ready been installed. Is there some way I can block this?
Posted by: Steve Prover at November 6, 2009 11:31 AMIn relation to auto updates I have experienced exactly the same as Lin Yu on exactly the same dates. This is the message I got today from the automatic update:- The following updates were not installed "Microsoft .NET Framework 3.5 Service Pack 1 and .NET Framework 3.5 Family Update for .NET versions 2.0 through 3.5 (KB951847) x86
Posted by: David Cromarty at November 19, 2009 8:08 AMMicrosoft .NET Framework 2.0 Service Pack 2 Security Update for Windows 2000, Windows Server 2003, and Windows XP (KB974417". Any ideas on a solution would be great. Thanks.
Well.. Thanks for this information as a regular software I thought 3.5 will take care of all the previous versions, but I was wrong it is still asking me to install 1.1 and 1.1 SP1.
This is crazy..But thanks Leo for this knowledge
Posted by: shanks at December 6, 2009 7:37 PMFor sometime now, on bootup, I get two dialog error boxes, both having to do with .NET Framework. The first one is titled NotifyAlert.exe -.NET Framework Initialization Error. This tells me to install one of the following versions of .NET Framework: v1.1.4322 or v1.0.3705. The second box is titled rng.exe - .NET etc. with exactly the same instructions to install one of versions. My list of installed programs have .NET Framework 2.0 Service Pack 2; .NET Framework 3.0 Service Pack 2; and .NET Framework 3.5 SP1. What do I need to get this all "fixed"?
Posted by: Murray at January 6, 2010 2:40 PMO.K. so if .NET Framework is so dang mecessary, why does it slow my computer down so much? This is really quite aggravating i mean cmon man, i have a 10GB hard drive and this is using 3,468.54Mb.
Posted by: mike b at February 5, 2010 7:46 AM