cannot install search hilite plugin in wp1.5.1

I’m running:

wp 1.5.1,
php 4.3.3,
MySQL 4.1.12,
apache 1.3.30

With activating the search hilite plugin via the plugin menu of the admin manager i get the following errors above the navpanel:

Warning: Cannot modify header information - headers already sent by (output started at /usr/www/webnojokes/blog.org/wp-content/plugins/search_hilite.php:140) in /usr/www/webnojokes/blog.org/wp-admin/admin.php on line 13

I’m using ff 1.04 with winxp-pro. This smells like a caching problem, but possibly i’m just to stupid to install.

Any help is greatly appreciated!