A.R.S.E.

I keep getting IE 'popunder' ads every 5mins or so...
I use Firefox.

Have tried the microsoft anti-spyware thing, also adaware.
neither of them find this crap to be able to remove it.

Anything else I can try?

A google search hasnt found me much either....

Spybot Search & Destroy: Linkage

HiJack This: Linkage

Don't use HiJack This unless you really really REALLY know what you're modifying/deleting. Spybot is usually more than enough to rid your pc of all spyware crap, ie; popups/popunders.

If all else fails, catch me on TeamSpeak, and I'll give you a brief rundown on HiJack This.

Cheers mate, good luck