MS Office documents suddenly become read-only [duplicate]

Possible Duplicate:
While editing a document in Word 2007 the document suddenly becomes read only

This is really annoying. When I work on a Word or Powerpoint document in MS Office 2007, after a while, the document becomes read-only by itself. As to how I learn this: When I try to save the document by clicking the save icon or using Ctrl+S, the "save as" window opens (this should not happen, the file should be saved directly). I select the same file name and try to save, but MS Office says (roughly):

Word cannot save the file. The file you want to save is read-only.

When I look at the file name at the top of Word window, "Read-only" is written next to the file name.

As a one-time solution, I save the file with a different name, and delete the previous (read-only) one. Then I rename the file to preserve its name.

I made an extra test: When the Word doc became read-only, I directly closed it without saving. Then I re-opened it, and it was NOT read-only. This is strange.

This happens with Word and Powerpoint, and it happens after some time of work. The computer does not sleep/hibernate in the meanwhile. I'm admin on that machine.

How can I fix this annoyance?

UPDATE:

I talked to the I.T. department. They told me they have received complaints from some other users on this issue and tried to solve this. So far, they couldn't find the reason and an exact solution. They also said this is not related with the network; this is a problem in MS Office.

CLARIFICATION DERIVED FROM COMMENTS:

1. The files are local and located on desktop and in "My Documents" folder. None of the folders on the PC is shared over the network. According to my experience so far, it happens to every MS Office file no matter where the file is located in the HDD after a certain period of time (after 15 mins, 30 mins, etc.). I googled this problem and I found out there are people having the same problem, but there is no answer.
2. This PC is on a company domain and our IT department's policies don't let us disable the AV software.
3. To-do: Create a new folder, put a word file there and then press Save several times. Does this "Read Only" bug appear or does it only appear once in a while? (Test result: No effect. It only appears once in a while)
4. To-do: Clean the temp folder.
5. I don't use any third party search tools like Google Desktop; so the problem is not related to file locking due to search indexing.

I would check for any processes like a virus checker or a network backup. Any process that accesses the file will fool office into thinking it is being shared, and thus revert it to read only until it can gain exclusive access again.