Event ID 537 is a generic logon failure that most of the time that I've seen it has a blank user name, to figure out what the true underlying cause of the logon failure you need to look at the Status Code and Substatus Code in the description. The codes that I see most often when talking to customers is:
Status code: 0xC000006D
Substatus code: 0xC0000133
These 2 codes indicate that the workstation clock is more than 5 mins out of sync with the Domain Controller. I have put together a blog entry on how to analyze event 537 that includes links to where to look up the Status and Substatus codes; ithompson.wordpress.com/2009/02/24/analyzing-id-537-and-the-status-codes/.
Hope this info helps.