Windows server reboots automatically

Hi All,

I am having one windows 2003 enterprise edition server with Citrix presentation 4.5 server installed. This server reboots automatically.
In event viewer it generates following events. Earlier also i was facing this error for that i have re installed the server but now again i am facing this error. Please help me to resolve this issue.

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Event ID : 1076 Source: USER32

The reason supplied by user server\Administrator for the last unexpected shutdown of this computer is: System Failure: Stop error
Reason Code: 0x805000f
Bug ID:
Bugcheck String: 0x000000c2 (0x00000044, 0x00000000, 0x80000000, 0x00000000)
Comment: 0x000000c2 (0x00000044, 0x00000000, 0x80000000, 0x00000000)

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Event ID : 1003 Source: System Error

Error code 000000c2, parameter1 00000044, parameter2 00000000, parameter3 80000000, parameter4 00000000.

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Thanks in advance,
Abhijit