Been confronted with this myself this week. I had .Net Framework 4 successfully installed on my laptop but this week the security update for it failed. (first time I have ever had an update fail on Vista)
I tried removing and reinstalling .Net Framework 4 only to find that it would not reinstall at all,even though it was installed previously - same symptoms as yourself accompanied the attempted reinstall.
I have removed and reinstalled all .Net Framework versions (as recommended by MS)with all updates etc and Framework No4 still will not install.All the others up to Framework 3.5 still install fine.
Done a lot of Googling on this and the general consensus seems to be that it is probably down to a third party program having corrupted the registry and that a complete OS reinstall may be the only solution (as it is virtually impossible to pinpoint what has done what to what and when)
Personally I'm not prepared to go through an OS reinstall just for the one program which I had which was looking for Framework No4 and I can live without it.
Sorry it's not more help but at least you know you are not alone and if you do come across a definitive answer that works .... please let us know.