Quite a few Vista users have had problems with McAfee Read through the threads below, there's a lot of info & fixes to try.

Vista + McAfee 2007 + LimeWire = dropping connection

“No Compromise” Vista-McAfee-Limewire Solution

Can't Connect With VISTA!


Some people have also had problems with Java 1.6.0_01. You could try this older version & see whether that makes a difference. Use the JRE offline installer & remove your current Java version before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11
Try the McAfee fixes first though, some people can run the latest Java update without a problem

If you end up removing McAfee, configure the Windows firewall to allow LW.
http://www.gnutellaforums.com/274003-post2.html

Do you have a router as well as the modem? That model of modem doesn't have an internal firewall but if there's a router as well...that could have a firewall.