Windows 7: Can't set up the network

Hey guys.
At the company we have a network. Some of them were connected with cables other laptops wireless! we had internet connection on all of them, plus the network between computers was arranged that all computers could see each other in the network (to share files, printers and so on).
Recently we changed the router and after that problems started to accrue. For the moment there is an internet on all machines but the network between them is messed up. Some of them see each other some of them not... (there are 2 PCs with windows xp, the rest are running windows 7).
They all are in the same Workgroup !!! Sharing options are ON, passwords are OFF! I don't know what else to do to recover this network, the way it was? maybe it's a problem from the router? I don't know
Pleas help me to figure this out guys. I would appreciate it a lot.
Thank you very much ))

Few Questions,

How many PC's are we talking. What ones cant see what and who can see who. Also ensure all the computers have the same "Workgroup" name. Maybe changing it on all PC's will solve this.

Also Check that the DHCP server is working 100% and all are on the Network Router.Ensure that there not assigned there IP's manually.

hey mrdemon. I appreciate your response to my question. I tried a little it more and now the situation is that all windows 7 computers can see each other but not Windows xp ones, and Windows XP ones can see all computers. I checked all Pcs are in a same workgroup!

Thanks

Hello
For a while the network was running very good. last coupl of days all the 64 bit machines lost the connection.
Have no clue why. no one touch any settings.
If you have any clue pleas help
Thanks