The Zone Labs firewall products Zone Alarm 5.0, Zone Alarm Pro 5.0, and Zone Alarm Security Suite 5.0 are not compatible with Azureus due to an issue with their TCP filter, TrueVector (vsmon.exe) Usual symptoms of this issue include but are not limited to 100% CPU usage, slow transfer speeds, corrupted files, trouble establishing incoming and outgoing transfers, complete network outages, complete system freezes, random reboots, and blue screens of death. Disabling Zone Alarm's firewall protection will not solve the problem as its filter will still be running in the background. A complete uninstallation is required. This issue is not limited to Azureus, and has been confirmed by users of many other filesharing clients including BitTornado, ABC, Shad0w's Experimental, EMule/Gnutella, and Shareaza.
Bottom line, don't use Zone Alarm 5.0. This is just one of the many many problems people are having with it. If you want examples, check their forums: http://forum.zonelabs.org/zonelabs
I've just switched to Sygate