Dropbox should be okay unless a shared file is the culprit.
Also, anti-virus is NOT anti-malware. You need two different software packages. I personally use Microsoft Security Essentials, which keeps the viruses at bay, and some malware. However, I use Malware Antibytes for my anti-malware.
http://www.malwarebytes.org/products/malwarebytes_free/With Malware Antibytes, I scan every so often (it's not real-time), and I especially scan when I think something is amiss. It generally catches whatever ails me. I've had to resort, on a time or two, to specific apps made by MacAfee, Symantec or some other security company to deal with a very specific malware.
Unless it's a real dinger, you should never have to restore to factory defaults. I've caught a few over the years, but haven't had to go down the system restore route, thankfully.
One tip I can offer: if you get any popup that LOOKS like a windows popup and says you have malware, do not fall for it unless it really is from your own anti-virus/anti-malware programs. No web site out there will legitimately "auto-detect" for you. This sort of thing is a scam and will royally screw up your computer. So, how do you know? It's hard to know, actually. It just comes with a little bit of experience and a gut feeling. If you wonder one way or another, simply close the box, then YOU initiate the scan with your own anti-virus or anti-malware programs. That way, if something is detected, you at least know it's legit because you started the scan instead of some web site saying you have a virus or malware.
Best
-LQ