You should take the time first to run through the Splunk tutorial here: http://docs.splunk.com/Documentation/Splunk/6.3.3/SearchTutorial/WelcometotheSearchTutorial This will give you some familiarity with how splunk works and how to get data into it.
In terms of how to achieve your audit that is really going to depend on what you need audited! If you're just getting a list of logins for two PCs I would probably suggest Splunk is overkill. Instead you can just connect to the event viewer on each PC and run a search.
If you do want to go down the Splunk route have a look at this prebuilt collection of inputs and dashboards: http://docs.splunk.com/Documentation/WindowsAddOn/4.8.1/User/AbouttheSplunkAdd-onforWindows
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