Lost network connections, internet OK.

I have 4 computers at 3 locations that have lost their connection to their servers. The internet works but no anti-virus/spyware will run or can be installed. The taskbar stays hidden and Malwarebytes gives error 372. All are XP professional with current SP3 installed. The had Norton or McAfee on and active/updated. Mapped network drives no longer available but server can be pinged from dos window.