The old logs are compressed and you can find these *.gz files in C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\Logs (default location).
One way to generate logs bundle is: Log into VC Server, go to drop down menu File > Export > Export System Logs....
Another way to find all vCenter logs is to generate a log bundle: Go to Start > Programs > VMware > Generate vCenter Server log bundle. This generates a zipped folder containing all vCenter Server related logs.
Thanks.
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