Vista - Acer PC slow sharing files and folders

This is driving me crazy.

We have a network with 6 PC's all running Vista, and 4 of the (2 Acer desktops and 2 Samsung Laptops) all share files fine. BUT we just added 2 new Acer 5535 laptops which WILL NOT share their drives nicely... though they will read other PC's drives just fine.

When connnecting to the Acer laptops over the network, they take aaaaaaaaages to respond, and client PC's often time out or hang.

I've tried:

• deleting the AV software... improved slightly but not much
• disabling the firewalls... no change
• wireless and wired connections... no difference

Settings in Network and Sharing are exactly the same as all the onther PC's...... I'm at my wits end. anyone got any ideas????