i have installed a windows 2000 computer on my lan, under vmware, then i have installed cain&abel, ethernet port 192.168.0.3

Cain&Abel is sniffing all my passwords (http, smb) on all my lan (192.168.0.2 0.4 0.5).

I would like to open it to DMZ but how can i disable the possibility for a hacker to sniff all my passwords, either they'd hack my vmware computer ?

you must have two network cards:
sniffer can find any data in network card and your vmware use the same as windows...

Yes, you need a seperate NIC dedicated for VMware guest OS if you don't want it a sniffer in the guest OS to sniff ALL the packets (i.e. for the host OS)