Windows User Login into Linux Server?

[WhyLinux]

We have an existing setup as Windows server 2003 with AD, with around 20 clients. All the clients are domain users with security policies and the client OS is windows XP professional.

All the clients use only one application. Now question is can we change our existing Windows server to a Linux server which can handle or allow login of XP users and security policy like WS 2003?

We cannot convert existing client from XP and the server is having a lot of problems and it's not worth our time. Is this possible?

[RHochstenbach]

You could implement an LDAP Directory Server on the Linux system. There are quite some guides about this online, but it's a rather complex topic.