my brother keeps going on sites like limewire even though he has been told not to as they are riddled with virises now he has got our pc infected with an istbar trojan how can i remove it ?

install and run

Anti Spyware s/w - SpyBot Search & destry or Ad-Aware

Virus s/w - McAfee or Norton

after having updated them fully

Then install something like netnanny so you can restrict the sites he can visit or just ban him from the computer

Couldn't you setup a restricted-rights User-login for him?

And perhaps lockdown/sandbox InternetExplorer as explained here.

Setup a virtual machine for him to destroy instead. That's what I do with all of my guests that connect to this machine. Since I test so many distributions with vmware, it also seems like a good tool to remove the boat from the leak.

Or just ban his a**.

That ISTbar is some nasty spyware. The real problems arise after getting rid of it, so I usually just reformat. =/

my anti virus avg finds it ok but does not remove it the trojans its found are called

ysbactive.dll & applet_y(2).cab

You really need to run the already suggested programs, last I used AVG it didn't have an option to flag a file to delete after next reboot. Or any utility that can add files to delete before the OS starts loading.

Like already mentioned some things even after being removed still have ill effects, like some of those desktop hijackers, even after being removed explorer will still attempt call-home outbound calls to ip-addresses - even with no trace of the software remaining.

First refer to this thread.

For a great free anti-virus for a year, get eTrust EZ Anti-Virus.

That should help with the issues at hand. Also, post a HijackThis log when you're finished.