problems conecting PC and Mac in home network

I have a PC running WinXP Pro and an old iMac on OS X Tiger. They were running fine for over a year then I got a virus and had to reinstall WinXP, using the same version. The internet works fine on PC and Mac via ADSL>USBModem>PC>LANCable>iMac. when I try and browse the network and open “workgroup” I get this message:

“WORKGROUP is not accessible. You might not have persmission to use this network resource. Contact the administrator of this server to find out if you have access permissions.

The list of servers for this workgroup is not currently available.”

I have no firewall on, it’s all set to DHCP. Can anyone help?

Cheers, Olly.