System alert ( malware)

Bonjour,
Un ami sembl avoir attrapé un malware ( après avoir imprudemment téléchargé un jeu, semble t-il ).
Il a une alerte ainsi libellée :
System alert System detected virus activities. These may impact the performance of your computer. Please, use recommanded antispyware software to protect your system from parasite programs
( N.B. : je l'ai dissuadé de le faire, bien entendu )
Il vient alors l'alerte suivante en fenêtre en bas à droite :
Worm Win31 Net booster detected on your machine. This virus is distribued via Internet through e-mail and Active-X objects the warm has its own SMTP engine which means it gathers e-mails from your local computer and re-distributes itself. In worst cases this worm can allow attackers to access your computer stealing passwords and personnal data.
This process should be removed from your system
Type virus
System affected Windows 2000,, NT, ME, XP, Vista
Security Risk Ca.S/.S
Recommandations click yes to remove it from your PC immediately OUI NON
(N.B. je lui ai conseillé de répondre non )
Que faire ? Merci