By moving to a new OS you closed some security doors but opened new ones.. Moving to a newer OS won't prevent it more than before.
You also need to know that all Antivirus work with 60-80% success hit so even with the shiniest Norton or other brand you will not be fully protected..
Now the good news:
Create 2 accounts, one is an Administrator like and the other one is the standard or restricted user account.
Always use the restricted account for surfing so the Virus and crap can't propagate to the whole system.
Also you can install a Virtual Machine software ie. VirtualBox and in there run a WIndows XP or whatever you want, and that is the one that you use to surf porn or whatever websites that have the highest risk (ask your favourite nephew or IT guy for advise on how to setup)
You can always move to a higher protection but we're talking thousands here..
Anyway, if your computer is fairly new then just reset it to factory and change the antivirus. I use the Microsoft essentials or whatever is called.. So far so good.
Try this version of the Microsoft virus scan (is not an anti-virus, just a scanner), this is what the internal people in Microsoft use:
Microsoft Safety Scanner - Free Virus Scan with the Microsoft Safety Scanner
Good Luck!