SharePoint

UserProfileManager userMgr = new UserProfileManager(ServerContext.GetContext(SPContext.Current.Site));

UserProfile profile = userMgr.GetUserProfile(userLogin);

SPUser (MOSS or WSS)

Get a User (SPUser) from the root Web site in the root Site collection of the current web application.

SPUser user = SPContext.Current.Site.WebApplication.Sites["/"].RootWeb.Users[loginName];

The EnsureUser method will return the SPUser object for the specified user login.  The user will be added to the site if the user does not already exist.  This will ensure an SPUser object is always returned.

SPContext.Current.Web.EnsureUser(@"domain\user");