Actually, i bet you are not deleting it properly!
Not only do these worms, viruses and trojans live within your system files, they also live within System Restore. If System Restore is not disabled before attempting to remove infections, they will automatically return when you reboot.
http://securityresponse.symantec.com/avcenter/venc/data/w32.sober@mm.removal.tool.html
Symantic has a tool especially to get rid of Sober at the above link.
1. Disable System Restore.
2. Read the instructions on the Symantic site.
3. Download the tool to your desktop.
4. Execute the downloaded tool.
5. Reboot.
6. Execute the downloaded tool again.
7. Turn System Restore back on if you are finally clean.
What are you running for AV software? Every decent AV package has been guarding against Sober since October of 03.
Let me guess, you don't have an AV package, you are using one of those free online scans whenever you think you might have a problem?!?
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The internet is no longer a toy, it's a COMBAT ZONE!