There's been a problem in recent weeks with the DNS servers of some ISP's being deliberately corrupted, with the result that attempting to access some popular web sites ends up with the computer concerned being redirected to a malware site that then downloads malicious software onto the local machine.
This then does things to the machine when it attempts to get to other sites, and can also try to steal access codes or passwords or both.
The best thing I can suggest at the moment would be to check that there's no spyware or similar on your machine, make sure that the temporary files area has been cleared, delete the recent history in Internet Explorer, and make sure that your anti virus and firewall packages are up to date. There's several packages suggested in messages here that can do these checks, and if you're not sure about the results, post them here for the more experienced users to view, and they will be able to confirm if there are any issues with the machine.
I can't be more specific on this one, as even the security people are having problems getting an exact handle on the DNS issue, both in terms of how widespread it really is, and in terms of the potential for damage to user machines, they know it's happening, but can't determine exactly where it's coming from, or the exact end result of the poisoning of the DNS servers.