Maybe you can try to download and install user32.dll from
this site
(remember to backup your existing user32.dll file before overwriting it with the one downloaded from there)
EDIT:
It seems that some people have problems with that dll file downloaded from ddl-files.com on Win2000 systems (somebody else don't have such a problem).
By searching the net I found several messages saying that that problem may be caused by a missing DirectX9 installation.
Download DirectX9 from Microsoft web site or throught LiveUpdate and the problem should disappears..