You may be able to access Domains and Domain Controllers using any kind of Windows: XP, Vista or 7, and also using their different types: home, prof or ultimate.
Indeed there are some comments or questions that needs to be fulfilled in order to answer your question.
IF, and IF ONLY your needs are to access Domain Servers, you can access them as explained using any kind of Windows. Please read the other post.
Holds a central database repository of usernames and passwords. All computers are authenticated within the domain, thereby granting access from a central point. This has increased security and helps manage multiple computers easily. This is a very basic explanation.
Within a domain, you authenticate against a domain server (domain controller). This will grant you access.
Independent computer setup. Anyone can access this computer remotely from another computer if they know a username and password on the server. Each person authenticates against their computer, unlike a domain where each user authenticates against a server. Workgroup servers are very basic, lack security, and can be a pain to manage in larger environments.
For more info look up the difference between a Domain and Workgroup.