iMac problem - I know it's not art, but ...

I have an iMac. I had trouble putting it in 'sleep', and actually couldn't open it again. So had to power off. When it came back, all my arial font had changed to an almost unreadable font. I couldn't read my browsers, so google searches were fruitless.
I opened up 'word' and copied the funky font from out of an email, over to there. It showed up as being Arial, but it wasn't.
I took Arial out of my font folder. It still didn't change.
I had to browse through all my fonts, looking for the Ag sample that matched. I ended up being 'JungleStandard.ttf'. So I took that out of the folder, and put the regular Arial back. That solved the problem, but immediately, in firefox I got a page coming up that showed my folders, and was something about iscan? It identified all these viruses, Trojans, and insisted I download an exe file immediately.
It was SCARY, and it wouldn't let me access other tags on Firefox. I couldn't close it.
As far as I know (I'm only a new iMac user), exe are not the program files for iMacs.
I was told I didn't need virus protection for my iMac.
I tried downloading clamxav, but couldn't work out how to do that.
I have downloaded a free virus checker. iAntiVirus - it's currently checking, and has not come up with any infections after about 15 minutes and 302,000 objects checked.
I'm also running trend on Safari, with no results showing. I'm not sure if it's doing anything.
Well, I just thought I'd report this in case anyone encounters funky font instead of arial on their iMacs. That scan thing was terribly realistic.