I and others had severe problems after installing KB890859. I was eventually
able to undo the problem but I found that I then experienced problems with
other Windows Updates and had to turn them off altogether on the machine in
question.

It's too late for you but the way to recover from this update is given at
http://loloyd.blogspot.com/2005/09/h...indows-xp.html

I followed this procedure but would suggest that once you can get some sort
of Windows functionality then a System Restore is the best option.