You do not mention whether you have VS2008 or VWD.  You can have either or both.  They should be at SP1; VWD will not even work without minimum of SP1 (i.e., current service pack level June/2009).  .NET Framework must be 3.5; that you have.  You should have no addins until after ASP.NET MVC v1.0 is installed.

Worst case scenario:  you may have to rebuild your system.  I had to face that because of a random bug.

If you do have to rebuild, follow this procedure after backing up anything you do not want to lose:
(1) install Windows
(2) update Windows until there are no more updates
(3) install VS2008 and/or VWD 2008.
(4) update VS2008 and/or VWD 2008 to SP1
(5) install ASP.NET MVC v1.0
(6) build the default ASP.NET MVC application before you do anything else.
(7) add back any VS addins you have [repeat step (6) after each addin]

Regards,
Gerry (Lowry)

References:
http://haacked.com/archive/2009/03/05/troubleshooting-installers.aspx
http://haacked.com/archive/2009/03/03/aspnetmvc-changes-for-rc2.aspx
http://forums.asp.net/p/1423139/3163091.aspx
http://forums.asp.net/t/1398843.aspx
http://forums.asp.net/p/1398714/3017831.aspx
http://forums.asp.net/t/1415432.aspx