cannot copy adsmsext.dll in service pack 2 update

When I try to install service pack 2 for xp I get the following
message'cannot copy adsmsext.dll
When I search for this file on my computer I find I have 3 on my system.
they are all on drive c and are as follows
c:\windows\system 32
c:\windows\servicepackfiles\i386
c:\softwaredistibution\download\lots of numbers

Being a computer novice I don't know which one to specify,if any,do I need
all 3 files?
Can anyone tell me what to do as I cant update at this moment

Many Thanks