I may not be a coincidence.
It is known that Google use information of what you search on to target advertising on web sites you visit.
So if you have been searching for "Digital Camera" it may be that other sites you visit include adverts for Digital Cameras.
More here
http://www.wired.com/politics/law/commentary/c ircuitcourt/2007/05/circuitcourt_0509
Also cookies keep track of web sites you visit and this information can be sent back to servers to help them target advertising.
If you look at a list of the cookes on your PC you will find them with names like googleadservices, doubleclick, webstat and so on.
All these people are keeping track of sites you visit and things you do so they can better track your online habits.
Also worry about gmail, the email system from Google.
They can scan the text of all the e-mails that you send a receive to see what "keywords" they can pull out so they can target advertising.
(Note they MAY have stopped doing this, but they do keep EVERY e-mail sent and recieved on gmail so they have a huge searchable database)
More here
http://www.gmail-is-too-creepy.com/
http://news.zdnet.co.uk/itmanagement/0,1000000 308,39151479,00.htm
If we all know how much we were being tracked it would frighten us.