Cannot access network

Got a problem accessing my network from my laptop. It runs Windows XP Pro SP2. The following occures when I try to access my W2K3 SBS-box:

ping servername -> works
ping serverip -> works
explorer \\serverip -> works
explorer \\servername -> no go
error: “No network provider accepted the given network path”

Things I tried:
Reinstalled TCP/IP (removed Winsock and Winsock2 entries in the registry, rebooted and forced a reinstall of TCP/IP)
Deleted NIC and reinstalled (only drivers)
Re-applied SP2 for Windows XP

All without effect.

Plz hlp