4
Xoops requirements:
WWW Server (Apache, IIS, Roxen, etc)
PHP 4.1.0 and higher (4.3.1 or higher for IIS)
MySQL Database 3.23.XX or 4.X.XX
You did not provide enough information (next time, please provide as much info as you could, you would get quicker and better answers), I suspect that your server is running PHP 4.0.6. With PHP 4.0.6, you would essentially be redirected back to the login page even you had the correct username and password.
I am only guessing. If that is not the case, take a look at the troubleshooting page and you might find your answer there:
http://wiki.xoops.org/wakka.php?wakka=TroubleshootingLoginProblems
