NET Framework is a swine.
You MAY have to install Windows XP Service Pack 3.
On XP with SP2, Windows Update installed a security patch, again and again and again.
I had to tell Update not to remind me.
The only solution was supposed to be to un-install and re-install.
I was on the phone with Micro-soft technical advisers several times using various techniques.
Every attempt met with failure, but no explanation or relevant error log.
Their final solution was to re-install Windows;
But Windows was supplied pre-installed without an installation disk, and they refused to give me one.
So I was stuck with a missing security patch.
Fortunately ESET and COMODO kept out any malware.
I found I could totally install NET Framework by use of
http://blogs.msdn.com/astebner/archive/200.../30/611355.aspx
That left hundreds of "missing dll" registry errors until I re-installed NET.
There remained about a dozen, either genuinely missing and needed, or counters gone wrong.
One thing I gained from that experience was a blow by blow log of everything done and errors encountered,
and that log file tells me the item which blocked a clean un-install,
and which the clean-up-tool removed by breaking the rules.
The problem was that some catalogue entry was missing,
and without that information the official un-installer did not know the correct sequence in which to remove items.
I restored a previous Disc Partition image to avoid those unknowns and tolerate a known security risk.
Subsequently XP SP3 was installed (for other reasons).
Some time later I find that the security patch is no longer missing.
SP3 did an awful lot of things I was unable to keep track of,
and I guess it fixed this for me as it went along.
Just possibly it might help you when all else fails.
Regards
Alan