What type of a server was it? Many times if physical servers are running hardware management and/or monitoring agents/tools and you migrate the server to ESX, you have to go through and uninstall all of the software specific to the old server when you bring up the vm. For example, any HP tools or utilities like ACU, Insight Diagnostics, System Mgt Agents, etc..
Try that first and if it doesn't work, you could go through and clean up all the non-present hardware after the P2V conversion. To do this go to a CMD prompt and type SET DEVMGR_SHOW_NONPRESENT_DEVICES=1 and then DEVMGMT.MSC and then select Show Hidden Devices. Delete any old grayed out hardware.
How many processors did your physical server have?
