Internet works but computers don't see each other - "MSHOME is not available..."

Hi, I have a problem with Sharing documents, and printers in a LAN (all comps Win XP home) network I am responsible for. It is a nice 32-port

1GBit switch, used by about 5-10 (sometimes more) computers at once. There was a long time it worked with no Internet (Sharing worked - slow - but worked). Then Internet was insstalled and everything worked fine, then Wifi was installed after some time, and again everything worked fine. And finally after a few weeks sth happened (don't know what) and Sharing doesn't work anymore. No, even worse. Two computers work with each other (sharing) but the rest says ("the MSHOME network is not available" - thou the internet works). I tried the XP built-in tool to configure the network - it says the network works. I tried static IP, and dynamic using DHCP, all the same. Ping works fine (