Microsoft themselves have denounced their child! Not really, but this seems quite a nasty little problem, so I thought I'd stick it here. Internet Explorer has a problem. It allows evil people to hurt you in some way. This will be fixed quite soon, but at the moment has not been fixed. It's more dangerous than usual, and has even hit the news.
To 'prevent' it, (this only applies to Internet explorer, other browsers are same as ever):
Tools >> Internet Options >> Security Tab >> Set security levels to "High".
There are other steps for beefing up your security against this threat, but they are complicated. Microsoft explain them in their original article here http://www.microsoft.com/technet/security/advisory/961051.mspx (under "Faq")
This applies to any site, and any site "may" be infected, even if it's one you regularly use (even here could potentially be!). There will be a patch soon, but better safe then sorry until then.