Otherwise, try boot up in Safe Mode and place a good copy of
dnsrslvr.dll into C:\Windows\System32 and any other directories that
contain that file.

If this doesn't work, attach the hard disk to another good computer
and do it from there. I wouldn't do any virus scanning this way
though, in case it deletes off important files and prevents XP from
booting up later. Once you're done fixing the file, reattach the HDD
back to the original computer, do a full virus scan before you install
SP3