Cannot create a text file

[Abhi]

I have a simple guestbook application which writes all the entries made on
the guestbook on a text file which I save at Request.PhysicalApplicatinPath
Now when I run the application it tells me that

Exception Details: System.IO.IOException: The process cannot access the file
"C:\MyPhysicalPath\guestbook.txt" because it is being used by another
process.

Then I tried changing the path to something else, well then I got an error
saying that MachineName\Asp.NET does not enough privilages to access the
path, well then I added the user ASP.NET to the adminsitrators group, but
still it says that the account does not have enough privilages.

Any clues what is going on ???

Abhi