"Did I now make my iMac more susceptible to viruses because I'm running PC software?"
No, as Windows malware can't affect Mac OS X, as it can't run, just like Windows software can't be simply executed within Mac OS X.
And
MS Office for Mac is not "PC (synonymous for "Windows" I suppose) software", it is Mac OS X software. Just because the company that makes MS Office also makes Windows, does not mean you use "PC software".
Currently there are
zero viruses affecting Mac OS X in public circulation, but there are other kinds of malware existing, that can infect your Mac.
But as long as you don't install software from unknown and untrusted sources, you are safe, as malware needs administrative permissions to run successfully, which means, you need to install the malware yourself, it can't install itself (one of the reasons, why a Mac OS X virus hasn't appeared yet).
To learn more about malware in Mac OS X and what steps can be taken to protect yourself, read the following F.A.Q.: