A virus called the Netsky.P worm was around earlier in the year is causing problems again, disguising itself as a Potter game, say anti-virus experts.
The virus, which first appeared towards the end of March, spreads by e-mail and via file-sharing services.
It is passed on when PC users open an e-mail attachment, so you need to be extra careful when using computers.
Sophos said they have spotted thousands of versions of the virus in the last few days.
The people who write viruses often use the names of cool celebs to try and convince people to open their creations.
Making the virus look like a Potter game is bad as the virus writers are hoping to take advantage of children using computers.
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