Why does my browser keep popping up dodgy adverts?

I have a problem whereby my browser sometimes pops up adverts, mostly for a dodgy debt consolidation company and an anti-virus program I've never heard of(which also does that annoying thing where it tells me it's going to ''scan'' my computer when I haven't installed anything in the first place, and have to press the close button). It's really starting to bug me and it has nothing to do with the websites I visit. How do I take off whatever's doing this?

Why does my browser keep popping up dodgy adverts?computer

Is the advertising trying to sell you, one of these useless products? It is scam advertising trying to swindle you. A computer infection called smitfraud is causing the phony warnings (see below).

AdwarePunisher,

AdwareSheriff,

AlphaCleaner,

Antispyware Soldier,

AntiVermeans,

AntiVermins,

AntiVerminser,

AntivirusGolden,

AVGold,

BraveSentry,

MalwareWipe,

MalwareWiped,

MalwareWipePro,

MalwareWiper,

PestCapture,

PestTrap,

PSGuard,

quicknavigate.com,

Registry Cleaner,

Security iGuard,

Smitfraud,

SpyAxe,

SpyCrush,

SpyDown,

SpyFalcon,

SpyGuard,

SpyHeal,

SpyMarshal,

SpySheriff,

SpySoldier,