Problems to connect one Computer with Vista Home Premium and three computers with WXP PRO SP2

I have problem with my Wireless home netwok based on Linksys Wireless router. It connects four computers (one cable and the other wireless) ) namely a DELL LATITUDE 501, a DELL Dimension 4550 (all with Windows XP PRO) and an ACER Aspire (with Windows Media Center 2005), all without any password and on the same workgroup , DHCP enabled, no password in any computer, Firewall disabled on all computers (just to try). Until I added the Vista Inspiron, all worked fine and now still keeps on working fine but only on the XP computers. They can locate the Vista computer on the map, but not access it and the Vista computer does not even succeed in locating the other three. (So the network is still on the three old computers)  The Vista (VIsta Home Premium)  can connect to INTERNET and get/send mail via the router without any problem and identifies correctly the router. The Inspiron has the same workgroup of the other ones  and according to instruction I set to "private" the nework on Vista and shared folders, printers, etc. The only thing that might be different is the subnet mask (provided that it has relevance), in the other computers is 255.255.0.0, here I don't know : frankly I did not find how to locate and modify it in VISTA. Which steps are needed to connect the new computer to the nework ? (it is WAP2 protected and of course  I entered proper network name and key).

Thanks in advance

Xiang Yan