If you spend enough time surfing the internet, you will eventually come across malware. It is usually part of a freeware download, or it can accompany an ActiveX download. Most of it is benign, but some is very aggressive, and cannot easily be uninstalled. It will continue to tax your system resources, and take up bandwidth, as it conducts it's business from your hardrive.

The best way to avoid trouble, is to assume it is there before you 'click'. The word FREE can be a very bad word. FREE toolbar, FREE screensaver, FREE music download, FREE plugin, FREE file-sharing software(peer to peer networks & illegal download sites), or FREE registry cleaner software. So if you are attracted to things that say FREE, be prepared to pay in another way. Time is valuable (unless you are a teenager) and you may spend the entire day trying to delete the malware from your system.

Community/fan websites like Train-sim, are not in the same catagory. Trustworthy websites have visible moderators, meaning, you can easily contact them, and they respond. Owners of unsavery websites don't offer contact info, that reaches them anyway.

In addition to avoiding free downloads online, NEVER click a link in an e-mail from an unknown source. You could be one click away from downloading a virus. By the way, some adware behaves like a virus, replicating itself during an attempt to uninstall it, or delete it. Once you have it on your system, there's no going back, only starting over.

Finally, running an anti-malware suite on your computer can really help, should you 'step in it' while surfing online.

Here's a list of good, 2nd generation security applications:

Trustware BufferZone Pro
PC Tools ThreatFire
DriveSentry

Anti-Spyware/Adware software:
Spy Sweeper

Anti-Malware Website:
Adware, Spyware and other unwanted "malware"
where they come from, and how to remove them cexx.org/adware.htm this is an extensive list