Its a windows trojan / spyware / dialer, although its on your system so ill assume you have had unauthorized users have access to your system somehow also..
Run chkrootkit and other similar apps to check your systems status, check your systems 'last' command to determine recent access to the system from users besides yourself also... you could check the date your /var/www/html/index.(index fille) was modified which may help you also.... have a look at your webmails source to check if if theres anything like the above code there
The main problem being, if its in your webmail and its in your systems main html its likely to be in a variety of other places also... and since its likely a cracker has gained access they could well have done more than simply adding bits of html, and its quite likely they have if they managed to gain access in the first place...
For your users (and probably yourself if you run windows) you will be wanting the following link for refernce:
http://computercops.org/modules.php?name=Forums&file=viewtopic&p=134881
Finally, the important thing with security, think before you fix... or you will make things far worse; basics first - lock down all external access (ssh/ telnet) besides your own IP lock down your firewall as tight as possible, check all running processes for anything unusual, check netstat to ensure there are no connections live besides standard http / ftp / mail /dns
Once you have done the above start doing system checks, use grep to check all users html for that code, check your /var/www/html files for it, phpmyadmin, uebimiau, squirrelmail etc
For the future you may also want to get something like TripWire (on your redhat disks for you!) which will notify you once the files are modified, a must for a situation like this
Good luck with it...
Chris