Falk AG, who serve the adverts for many a site on the Internet, The Register included, had one of their load balancers fiddled-with a couple of days ago. Any IE users not on XP SP2 may have been exposed to some nasty code as a result.

Bofra/IFrame is a currently unpatched exploit which affects Internet Explorer 6.0 on all Windows platforms bar Windows XP SP2. If you may have visited The Register between 6am and 12.30pm GMT on Saturday, Nov 20 using any Windows platform bar XP SP2 we strongly advise you to check your machine with up to date anti-virus software, to install SP2 if you are running Windows XP, and to strongly consider running an alternative browser, at least until Microsoft deals with the issue.
Check out the initial notice here and a statement from Falk here.