metropolitan police trojan on son's laptop

[vix]

hi....got another post up re my avast but son showed me his laptop yesterday, which is infected by something claiming to be the metropolitan police, need to send £100 to fix, etc.  obviously a trojan.  he's got macafee on it but it has clearly failed somehow.  I tried to download avast a few times on it yesterday but it stops at the last moment and is incomplete.  Did an online scan with something else and it says there are files it cannot read. 
any assistance in getting around this particular trojan gratefully received.
thank you.

[Rohugh]

You need to be posting in the Viruses and Worms forum for help.

http://forum.avast.com/index.php?board=4.0

[Pondus]

I tried to download avast a few times on it yesterday but it stops at the last moment and is incomplete. 

bc you already have a AV installed...
never install multiple AV as this will give a slow machine, windows errors and false detections

as already said, start a topic in virus and worms forum section where you follow this guide.   http://forum.avast.com/index.php?topic=53253.0

then help will arrive

[vix]

thank you!