I updated my Tiger-Parallels 5160-System with Leopard. After that I could no longer connect to my Microsoft network with Leopard (XP and Server 2003 computers)! Sometimes I could connect, the next second the connection was gone! In most cases the connection failed. After uninstalling Parallels the network worked fine. I'm really pleased!
balkinboris, Thanks for the tip. I'm running Parallels 2.5 Build 3124 on Leopard, ran into the same networking issue, did what you suggested and all is good.