Blank Page after Login, WP 1.5

I have tried every solution expressed through the support pages and the codex, and still can’t seem to solve this problem:

I installed a plug-in two days ago, Wordspew Ajax Shoutbox, and although it worked great, after installation, anyone attempting to login (there are 6 users) received a blank page. I was able to get to the admin panel by going to http://www.doomsdaylaser.com/wp-admin, and quickly disabled the plugin. That didn’t work. I then deleted the plugin through FTP and that didn’t work. I even uninstalled the plugin with a handy file that came with the plugin, which deletes the database it created, but that didn’t work.

Then, the moment I deleted cookies on my computer, I was unable to get back to the admin page. I have deleted (yes completely deleted) and re-uploaded several files as instructed on the codex and many other support pages, but nothing has worked thus far. Any suggestions? Thanks so much! oh, and my site is http://www.doomsdaylaser.com and I have PHP 2.5.2-pl1 and mySQL Version 4.1.11. The various users on the blog use both IE 6 and Firefox.