As of now there are no malware threats against Macs, and you should remain adequately protected by keeping your OS properly updated. Malware protection is built-in OS X, and it is updated as long as you are online or update your OS X software.
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
I don't know if it's possible for a trojan to access my MacBook Pro or my iMac.
Your profile doesn't indicate what OS X you are using on your MBP or iMac. The newer the better, but you need at least a fully up-to-dat OS X 10.6.8 to be protected against currently known Mac malware.
On the other hand, I don't know of any Trojan that would act in this way let alone interact with a PC.
What exactly does it mean that her Amazon account "showed [your] email address as having accessed her Amazon account"? I have the feeling that there was quite a lot of information lost somewhere between the tech and you, so you may not be able to clarify that statement, but it doesn't make a whole lot of sense as-is.
In any case, her having an infection on her Windows machine does not in any way imply anything about your machine. Whatever is going on with her machine, it's not indicative that you are infected with anything.