I have a Windows Server 2008 domain with both WinXP and Win7 PCs. Users are usually fine to log in to either OS, but there is one user who can't login on XP. It fails to create his profile. I believe they are mandatory profiles. How can I allow this user to log in to either Windows 7 or Windows XP?
Your question doesn't make a whole lot of sense in its current form. From what I can ascertain, you have roaming profiles or mandatory profiles configured and a profile that works on XP doesn't work on Windows 7.
This is because Windows 7 (and Vista and Windows 8 as well) use a new profile structure. You cannot use XP-era profiles with Vista, 7, or 8 clients.
There is a fair amount of info on this topic on Technet and in the Technet forums. I am not going to replicate it, but here is a forum question with a ton of good info.
What I will say is that you should use this as an excuse, if possible, to eliminate roaming and mandatory profiles. Use folder redirection instead. With XP clients it requires SP2 or SP3 and the GP Client-side Extensions on each workstation.