Can windows virus/trojan/spyware affect OSX/Unix/Linux systems?

No, as those are written for Windows, which is completely different than Mac OS X or any Linux variant.
Maybe this can help you:

Mac Virus/Malware Info by GGJstudios
There are currently no viruses for Mac OS X in public circulation, only a handful of trojans and other malware, which have to be installed manually via entering the administrator password.
The only anti-virus you need to protect your Mac is education and common sense.​

 

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If you use an emulator like Wine that can run Windows apps, you run the risk of hurting your system if you are not careful. Not that it can infect your whole system, but it can cause damage.

But as a general rule, you can't run Windows programs on non-Windows systems.

 

@OP: I used to used Intego security software, but it requires one to update a licence each year and that was a pain. Also, once I decided not to use it, it was very difficult to uninstall - tracking down the various background processes and files it spawned was frustrating. I also tried free antivirus software from Sophos. However, I used many programs that are based on JAVA and Sophos used to go into a little spasm each time one of those programs cranked up, slowing things down.

After reading various threads in MacRumors, I now use ClamXav. As for damage from Windows viruses, I believe the main threat is simply inadvertently acting as a conduit to transfer viruses to Windows machines, but there is the possibility that Windows malware could damage your system if you are running a Windows emulator in MacOS. I use Parallels Desktop to run a windows virtual machine, so I make sure that it has anti-virus software (my university has a site licence for F-Secure, which is OK) and that Windows is updated each time I log on.

 

They should, since they're at far greater risk of infection from sources other than files received from Mac users.

No, it's not. It's each user's responsibility to protect their own computers. To expect others to provide that protection is irresponsible, since you can't control what others may do.

Exactly! So every Windows user should be protected by their own AV software. However, AV software isn't required to protect a Mac from malware.

Given your user name, I wouldn't expect you to offer unbiased recommendations.

Sophos is not recommended, as it can actually increase your Mac's vulnerability.