Problems with pop-ups

on various sites I get messages about how my PC is under threat (even though I have a Mac)

No viruses that can attack OS X have so far been detected 'in the wild', i.e. in anything other than laboratory conditions.

That is 'scareware' trying to lure ignorant Windows users into downloading and installing malware. The sites you have visited have been hacked, and in the process you may have acquired a 'tracker cookie'.

More on that here:

http://news.bbc.co.uk/1/hi/technology/8313678.stm

Beware of PDF files from unknown sources. A security firm announced that by its counting, malicious Reader documents made up 80% of all exploits at the end of 2009.:

http://www.computerworld.com/s/article/9157438/RoguePDFs_account_for_80_of_all_exploits_saysresearcher

It is possible, however, to pass on a Windows virus to another Windows user, for example through an email attachment. To prevent this all you need is the free anti-virus utility ClamXav, which you can download for Tiger and Leopard from:

http://www.clamxav.com/

The new version for Snow Leopard is available here:

http://www.clamxav.com/index.php?page=v2beta

( Note: ClamAV adds a new user group to your Mac. That makes it a little more difficult to remove than some apps. You’ll find an uninstaller link in ClamXav’s FAQ page online.)

However, the appearance of Trojans and other malware that can possibly infect a Mac seems to be growing, but is a completely different issue to viruses.

If you allow a Trojan to be installed, the user's DNS records can be modified, redirecting incoming internet traffic through the attacker's servers, where it can be hijacked and injected with malicious websites and pornographic advertisements. The trojan also installs a watchdog process that ensures the victim's (that's you!) DNS records stay modified on a minute-by-minute basis.

You can read more about how, for example, the OSX/DNSChanger Trojan works here:

http://www.f-secure.com/v-descs/trojanosxdnschanger.shtml

SecureMac has introduced a free Trojan Detection Tool for Mac OS X. It's available here:

http://macscan.securemac.com/

The DNSChanger Removal Tool detects and removes spyware targeting Mac OS X and allows users to check to see if the trojan has been installed on their computer; if it has, the software helps to identify and remove the offending file. After a system reboot, the users' DNS records will be repaired.

(Note that a 30 day trial version of MacScan can be downloaded free of charge from:

http://macscan.securemac.com/buy/

and this can perform a complete scan of your entire hard disk. After 30 days free trial the cost is $29.99. The full version permits you to scan selected files and folders only, as well as the entire hard disk. It will detect (and delete if you ask it to) all 'tracker cookies' that switch you to web sites you did not want to go to.)

Did you actually read my post?

Download and use http://www.clamxav.com/