now I am getting this error:
Parse error: syntax error, unexpected T_PUBLIC in H:\www\wikipedia\extensions\CheckUser\CheckUser.hooks.php on line 63
It looks like your version of this file is missing some things; did you download using Subversion?
Then, if you're using MediaWiki 1.18.0, you'll have to use Special:ExtensionDistributor/CheckUser to download a 1.18-appropriate version. The version you downloaded is likely a late version with things not implemented yet in 1.18.