weird problem with an excel file, asp.net and IIS

Hi,

i have a weird problem and i don't know who is responsible for this: IIS,
excel or asp.net.
My problem:

we use an asp.net 2.0 application under IIS 6.0 (server 2003 sp2) which must
write data from a form into a table in excel files (i know excel is not
really recommended for that, but it's excel).There are a lot of excel files,
all in the same directory with the same privileges (Network service has
r/w).

There is only a problem with ONE excel file, where it's not possible to
write data (Operation must use an updateable query) and that file is locked
(can't open it) unless i delete the w3wp.exe process.

Can that excel file be 'corrupt' or something? When i open it, there is
nothing abnormal.
Where have i to seatch?
Thanks for advices
Ben

The whole message i get in the Event log is here:

Application information:
Application domain: /LM/W3SVC/1/Root/enquete-2-128275414437394676
Trust level: Full
Application Virtual Path: /enquete
Application Path: C:\Inetpub\wwwroot\enquete\
Machine name: SRV754

Process information:
Process ID: 3716
Process name: w3wp.exe
Account name: NT AUTHORITY\NETWORK SERVICE

Exception information:
Exception type: OleDbException
Exception message: Operation must use an updateable query.

Request information:
Request URL: http://srv754/enquete/ros2rp0607.aspx
Request path: /enquete/ros2rp0607.aspx
User host address: 10.135.181.130
User: CAMPUS\L0386
Is authenticated: True
Authentication Type: Negotiate
Thread account name: NT AUTHORITY\NETWORK SERVICE

Thread information:
Thread ID: 1
Thread account name: NT AUTHORITY\NETWORK SERVICE
Is impersonating: False
Stack trace: at
System.Data.OleDb.OleDbCommand.ExecuteCommandTextForSingleRe sult(tagDBPARAMS
dbParams, Object& executeResult)
at System.Data.OleDb.OleDbCommand.ExecuteCommandText(Object&
executeResult)
at System.Data.OleDb.OleDbCommand.ExecuteCommand(CommandBehavio r
behavior, Object& executeResult)
at System.Data.OleDb.OleDbCommand.ExecuteReaderInternal(Command Behavior
behavior, String method)
at System.Data.OleDb.OleDbCommand.ExecuteNonQuery()
at enquete.Page_Load(Object sender, EventArgs e) in
C:\Inetpub\wwwroot\enquete\enqueteG2.aspx.vb:line 20
at System.Web.UI.Control.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)