Cannot Install Net Framework 3.5

I have been running net framewo开发者_JAVA技巧rk 4.0. When I look at IIS, I can see that I have 1.0, 2.0 and 4.0 installed. I have to install 3.5 because I have a web site that is not ready to upgrade to 4.0.

When I try to install 3.5 I get an error, right after I accepting the agreement, it says download complete and then setup error.

Any thoughts? Can I install 3.5 over 4.0.

Thanks in advance.

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If you're just looking in IIS, you will never see ASP.NET 3.0 and 3.5.

.NET 3.0 and .NET 3.5 never updated the ASP.NET version -- they were extensions to .NET 2.0

Here is a post on Scott Hanselman's blog explaining this.