Tried installing IIS 5.1 on a freshly formatted Windows XP SP2 laptop.
Setup cannot copy the file staxmem.dll
…insert “Windows XP Professional Service Pack 2 CD”

Wonderful.
Took about 15 minutes on Google to find this:

esentutl /p %windir%/security/database/secedit.sdb

Got it from here:
http://www.ilovett.com/blog/archives/2004/10/03/dealing-with-staxmemdll/
He says it best... "Why didn't I think of that?"