XP connecting to W2k Advanced Server over Wireless Router

I have a home network setup (and it used to work until recently); The network
config is as follows: 1). A laptop with WXP Professional, 2). A PC with WXP
Professional, 3). A PC with W2K Advanced Server. All 3 connected to Internet
via wireless router without any problem. The Home XP PC serves as the Print
Server, the Home W2K PC serves as the File server. It used to work without
any problem. Now, the Home W2K can see and access any XP machine. The XP
Laptop and PC can Ping the W2K PC, but the XP machine can NEITHER SEE NOR
CONNECT to the W2K PC now. My kid recently reinstalled the W2K PC and with
all logon user name reinstalled. So what gives?
Any help is appreciated.
Sam