Well I know that windows xp professional services pack2 is not sporting more then 10 users connect at the same time. I am using windows xp professional in a small more then 35 user organization as a server but a 11 person is trying to acess he can not acess it. Can any body please help me where I download a patch that can be support more then 40 users . Because if I am going to change the operating system of server I have to change hole organization structure.

Well for starters you have answered you own question.
Also why are you even thinking about using a DESKTOP operating system as a server for this number of users.
As a minimum you should be using a SERVER OS such as 2000 or 2003.
This would completely alleviate your current issue and introduce some much needed security.

You could even consider using linux, such as Ubuntu for your server needs.

Also, Microsoft Small Business Server would be a good fit. It has a 100 user limitation but I that is more than enough and it is less expensive than the Server Standard or Enterprise versions.

Also, at a reduce rate, you can get the MS Exchange and MS ISA server options with SBS to add a corporate email server and Internet Proxy to the mix.