I have a small office and running about 9-10 pc in a room including fax/printer. I can access all pc using wireless router. But now I got a new issue.

We have a invoice system which is installed on one machine using wamp server. What I want to do is to access this invoice system from all other machines. Is this i can achieve or it is not possible.

It is possible if you create a network between all of the PC's. Then place all the information in a Shared Folder on the invoice computer. Then you should be able to access that information from any one of the other computers.

It is very possible,Since you have a wireless router in the office and you can access all the Pc .Then I think what you should do is just to create a folder on the system and share the file on the invoice system.
I think it should work that way.

a network is already created if you can access all the PC's. I would create a different user to be able to login remotely, since you wouldn't want to give everyone access to the server. Like stated above, you have to share a folder in the server so the other computers can access it.