BSOD several times a week

I'm having the following issue. Server 2003 crashes 2-3 times a week. Time in most cases is between 1500-1700 at afternoon. In event viewer I can see the BSOD event:

Source: EventLog
Category: None
Description: The previous shutdown at "time and date" was unexpected.
System is running Citrix access essentials, I do not know if it is related to the problem.

System is in VMWare ESX, here are details of the machine:
Server 2003 R2 Standard edition SP2 32-bit
Intel Xeon CPU
3,75GB RAM
Vmware tools are up to date.

I tried to workaround this problem with scheduled task that reboots the server every night, but it did not help.

Three latest Minidumps are attached (zip and individual files), but I cannot locate the problem from there.

View attachment

View attachment Mini041212-02.dmp
View attachment Mini041012-01.dmp
View attachment Mini041212-01.dmp

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