How do i stop mails generated by email spoof ?

Now a days i am receiving lot of spam mails generated by my id to my id. I have Exchange 2003 Server and Outlook 2007 client. I scanned my laptop for viruses, spyware, malware but it is not infected. My observation is that even when my laptop is off and when i switch it on, in my mailbox i can see lot of spam mails which are generated by my id, sent time of these mails was when my laptop was switched off. Can anybody help me with a permanent solution to get rid of this problem ?
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Dear Sir,
There is no real way to stop spoofing except to delete the messages as they arrive. To accomplish this, you can simply define a filter that will delete messages based on specific values in the email message header. In our case, we set the header to delete messages sent from the MAILER-DAEMON address.
If you need to handle a specific type of e-mail message in a special way, you may want to customize some message filters on Outlook. This will ensure that messages tagged as an example: "SPOOF" will be placed in your junk mail folder.  
1.     Open Outlook and Click on the Tools drop down menu and select Rules and Alerts ...
2.     In the window that opens, click on the New Rule... button.
3.     In the Rules Wizard dialogue box, under Step 1: Select a template, select Move messages with specific words in the subject to a folder.
4.     Under Step 2, click on the blue, underlined phrase, specific words.
5.     In the window that opens, type the words (example: "Spoof") in the text box at the top.
6.     Click Add.
7.     The new search criterea will appear under Search list:. Click OK.
8.     Under Step 2, click the blue, underlined word, specified.
9.     Select Junk E-mail and click OK.
10.   Click Finish.
11.   Click OK to enable this rule for all future mail handling.
chelpoolunitedAuthor Commented:
Dear Ahmed,

Thanks for your help. But I look at it as a work-aroud and not a solution. I beleive there has to be some solution for this problem. My worry is that today i am facing this issue with one id in my organisation, tomorrow if it happens with others then my mail flow will definately affected.
Dear Sir,
Another Solution is to enable Sender ID filtering in Exchange 2003, Sender ID works by verifying every e-mail message indeed originates from the Internet domain from which it was sent. This is accomplished by checking the address of the server sending the mail against a registered list of servers that the domain owner has authorized to send e-mail.
Please refer to the following article to configure and enable Sender ID filtering in Exchange 2003:

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