When launced by using systray, OCS does not produce any log and there is an application log in event viewer in windows7 as in the following:
 
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
- <System>
  <Provider Name="OCS Inventory Service" /> 
  <EventID Qualifiers="0">20</EventID> 
  <Level>2</Level> 
  <Task>0</Task> 
  <Keywords>0x80000000000000</Keywords> 
  <TimeCreated SystemTime="2011-10-11T07:29:03.000000000Z" /> 
  <EventRecordID>17332</EventRecordID> 
  <Channel>Application</Channel> 
  <Computer>xxxxr</Computer> 
  <Security /> 
  </System>
- <EventData>
  <Data>Can't launch OCS Inventory NG Agent (敇⁴佃卍䕐⁃牅潲㩲䔠牲牯挠摯⁥〲″‽桔⁥祳瑳浥挠畯摬渠瑯映湩⁤桴⁥湥楶潲浮湥⁴灯楴湯琠慨⁴慷⁳湥整敲⹤†)</Data> 
  </EventData>
  </Event>

But if I run the agent by executing OCSIncentory.exe, theOCSInventoryLog file's content is:

==============================================================================
Starting OCS Inventory NG Agent on Tuesday, October 11, 2011 10:31:35.
AGENT => Running OCS Inventory NG Agent Version 2.0.0.24
AGENT => Using OCS Inventory NG FrameWork Version 2.0.0.24
AGENT => Loading plug-in(s)
	PLUGIN => Searching for Plug-in DLL(s) in folder <C:\Program Files (x86)\OCS Inventory Agent\plugins>
	PLUGIN => 0 Plug-in(s) succesfully loaded
AGENT => Using network connection with Communication Server
	COM PROVIDER => Loading Communication Provider <C:\Program Files (x86)\OCS Inventory Agent\ComHTTP.dll>
AGENT => Using Communication Provider <OCS Inventory NG cURL Communication Provider> Version <2.0.0.24>
AGENT => Sending Prolog
	DID_CHECK => Read DeviceID <xxxx-2011-07-04-14-25-55> and MACs <70:F3:95:0E:01:07> in file <ocsinventory.dat>
	COM SERVER => Initializing cURL library for sendRequest
	COM SERVER => Using cURL without server authentication
	COM SERVER => Disabling cURL proxy support
	COM SERVER => Disabling cURL SSL server validation support
	COM SERVER => Sending HTTP Post request to URL <http://xxxx/ocsinventory>
	COM SERVER => HTTP Post response received <HTTP Status Code #200>
	COM SERVER => Cleaning cURL library
AGENT => Prolog successfully sent
SUPPORT => No support detected, Registration key : N/A
ERROR *** AGENT => Failed to write Prolog Frequency <Error code 32 = The process cannot access the file because it is being used by another process.  >
AGENT => Inventory required
AGENT => Launching hardware and software checks
	INVENTORY => Loading Download history
	INVENTORY => Logged on user ID is <xxx>
	INVENTORY => Operating System is <Microsoft Windows 7 Professional 6.1.7601 Service Pack 1>, description <>
	INVENTORY => Operating System uses 64 bits memory address width
	INVENTORY => Computer domain or workgroup is <xxx>
	WARNING *** INVENTORY => Failed to retrieve user domain
	INVENTORY => System Manufacturer <LENOVO>, System Model <5536AS1>, System S/N <R8H90D2>, Bios Manufacturer <LENOVO>, Bios Date <11/25/2010>, Bios Version <5JKT53AUS>
	INVENTORY => 1 processor(s) Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz [2 core(s) x64] at 3193 MHz
	INVENTORY => OS Memory 3766 MB, OS Swap size 7531 MB
	INVENTORY => 4 memory slot(s) found
	INVENTORY => 2 input device(s) found
	INVENTORY => 24 system port(s) found
	INVENTORY => 5 system slot(s) found
	INVENTORY => 3 system controler(s) found
	INVENTORY => 2 storage peripheral(s) found
	INVENTORY => 2 sound device(s) found
	WARNING *** INVENTORY => Failed to retrieve modems
	INVENTORY => 1 network adapter(s) found
	INVENTORY => 10 system printer(s) found
	INVENTORY => 2 video adapter(s) found
	INVENTORY => 1 system monitor(s) found
	INVENTORY => Default IPv4 address is <10.13.55.157>
	INVENTORY => Registered company <>, registered owner <xxx>, Product ID <55041-OE...>
	INVENTORY => Product key <237XB-GD...>
	INVENTORY => 193 software found
	INVENTORY => Computer/VM UUID is <64AA7664-B768-CCAA-66C9-AAF80417AB20>
	INVENTORY => Computer seems to be physical host
	INVENTORY => Bogus summed Memory Slots, 4096 is too far from 3766 (keeping the last one)
	INVENTORY => Reading last inventory state
AGENT =>  Communication Server ask for Registry Query
	REGISTRY => Executing query asked by server
	REGISTRY => 5 query successfully executed
AGENT =>  Communication Server ask for Package Download
	DOWNLOAD => Package history file successfully cleaned for duplicate IDs
ERROR *** DOWNLOAD => Can't create OCS Inventory NG Download process (	EXECUTABLE PLUGIN => Searching for VBS script(s) in folder <C:\Program Files (x86)\OCS Inventory Agent\plugins>
	EXECUTABLE PLUGIN => Searching for executable(s) in folder <C:\Program Files (x86)\OCS Inventory Agent\plugins>
AGENT => Sending Inventory
	INVENTORY => Checking last inventory state
	INVENTORY => Logical drives inventory state changed
	INVENTORY => Software inventory state changed
INVENTORY => Inventory changed since last run
	INVENTORY => Generating XML document with Device properties
	INVENTORY => XML Update 4 Logical Drive(s)
	INVENTORY => XML Update common Device properties
	INVENTORY => XML Update BIOS
	INVENTORY => XML Update 2 Input Device(s)
	INVENTORY => XML Update 24 System Port(s)
	INVENTORY => XML Update 3 System Controler(s)
	INVENTORY => XML Update 5 System Slot(s)
	INVENTORY => XML Update 2 Sound Device(s)
	INVENTORY => XML Update 2 Storage Peripheral(s)
	INVENTORY => XML Update 3 Logical Drive(s)
	INVENTORY => XML Update 0 Modem(s)
	INVENTORY => XML Update 1 Network Adapter(s)
	INVENTORY => XML Update 2 Video Adapter(s)
	INVENTORY => XML Update 1 Monitor(s)
	INVENTORY => XML Update 10 Printer(s)
	INVENTORY => XML Update 194 Software
	INVENTORY => XML Update 10 Registry Value(s)
	INVENTORY => XML Update Administrative Information(s)
	COM SERVER => Initializing cURL library for sendRequest
	COM SERVER => Using cURL without server authentication
	COM SERVER => Disabling cURL proxy support
	COM SERVER => Disabling cURL SSL server validation support
	COM SERVER => Sending HTTP Post request to URL <http://xxxr/ocsinventory>
	COM SERVER => HTTP Post response received <HTTP Status Code #200>
	COM SERVER => Cleaning cURL library
AGENT => Inventory successfully sent
	INVENTORY => Writing new inventory state
	WARNING *** INVENTORY => Failed to write new inventory state to file <C:\ProgramData\OCS Inventory NG\Agent\last_state>
AGENT => Unloading communication provider
AGENT => Unloading plug-in(s)
AGENT => Execution duration: 00:00:06.