Go to the 'Virus and Security Alerts' and read through
the countless posts about the 'about:blank' virus. It seems to be a nasty one. The forum search (on the main forum page) might be useful also.

If you happen to post again, note the following tips (just in general):
- mention OS
- mention browser (name and version)
- mention the full and exact text of the message
- mention when the message appears

Hope this helps.