Vista - Mapping Drives with Vista and XP Pro SP3

Hi,

I have a little problem and hope someone can point me in the right direction. Here is my situation and problem :

3 Computers on the Network ( all same subnet and workgroup ) :

2 x Win XP Pro SP3 Computers
1 x Win Vista Ultimate SP1

I have on my Vista machine a mapped drive to one of the XP machines and everything works just fine. Shows up in Vista Explorer, no problems at all. Now I wanted a second mapped drive to the second XP computer and I can't map the drive at all. When I open Network and sharing Center and click on "View full map", the computer name of the second XP computer shows up, but I can't see the drive and can't map it.
However, this second XP Computer is perfectly able to access my shared Vista drive without a problem. So the network configuration should be fine.

So, I can see and map in Vista Explorer ONE WinXP computer, but not the second one. Any ideas ?

PS: ALL computer have all available updates installed and are up-to-date.

It seems like on the second computer, a service or something isn't running, and therefore not allowing my Vista computer to connect. But what service could it be ?

Thanks for helping !

Cheers,

JCE