Host Only Networking Issue

I am new to Parallels, so this might be something simple that I am overlooking......I have a recurring problem were my VM's become isolated when running in host only mode. I am running three VM machines: 2 CentOS, 1 redhat enterprise. The CentOS machines boot fine, and I can ping back and forth btw the 2 VM's - but not from the host to the VM's nor from the VM's to the host. Additionnally, when I launch the Redhat machine I get
eth0 failed --- even when i go in an manually assign/bring up eth0 I still cannot ping any of the machines.

When I initially loaded Paralles my systems work perfectly ---- then i started to get an error -- "ApiInit: Cannot allocate buffer for packets" and all of the machines would be isolated as described above. Support had be re-install Parallels -- I no longer get the error message, but my machines are still isolated.

Any help would be greatly appreciated!

Host OS - Vista Business
Host machine - Latitude D630