Your homepage has been hijacked. Do you have AdAware or spybot? If so run them, if not they are availlable on PC Advisor cover disks. Also make sure your Antivirus is up to date.
Once you have run them then for an extra bit of security use system restore to go back to a date before the problem started.
Seems the 'about blank' is doing the rounds at the moment, I had it and cured it using the above method.
Once you have done the above then go into control panel > internet Options and make sure your home page has been restored, if not then re-set it.