It appeared immediately after clicking a google search result to another forum that I hadn't been too before.
Usual windows style window appearing with lots of progress bars and stuff about active attack happening from the internet, etc etc. Pressure to click "activate now" button for software called "Malware Protection"
It was quite irritating because it disabled task manager, and I couldn't open a browser or run my AV tools. After a while this stopped but there was still an icon in the system tray. System performance seemed a lot slower so it might be interfering with this.
I have another pc to use, so can work this through over a period of time but my actions so far have been:
System restore to yesterday
Running the Microsoft tools linked to in another thread on here
Changed my account to a standard rather than an administrator
Ran CClean and Reg cleaner
I'll see if I can identify the thing from there and remove any traces