Vista SP1 & XP Pro - Networking Problem

I have a network of 10 pc's in a workgroup: 7 Vista Home Premium & 3 XP Pro.
Prior to the SP1 release, Vista and XP were playing nicely; the XP machines
have drives mapped to one particular Vista machine's shares, with no
problems. However, ever since installing Vista SP1, Vista & XP Pro have NOT
been playing nicely. The same mapped drives are unable to connect to the
Vista shares, and pinging the Vista machine doesn't even work. From the
Vista machine, the XP machines appear in the Network map but I'm not able to
open them.

Can anyone explain to me what the heck Vista SP1 did, and what I can do to
fix it?

Thanks!
Travis P.