Windows Vista SP1 lose connectivity in 2008 Domain

We have a flat network of 15 PCs an 4 Domain Controllers running Windows 2008. The PCs are Windows Vista Business Edition SP1. The network is flat, meaning the PCs connect to the Servers, with a cisco switch wia a trunk port and the to the Internet. the network works fine, except one particular PC, that constantly loses connectivity to the network. The DNS and DHCP are working perfectly, so I don’t think there is an issue from there..Could someone has an idea why this problem constantly occurs??