If this is happening on more than one website, your computer is definitely
infected with something.

Go through these general malware removal steps systematically -
http://www.elephantboycomputers.com/page2.html#Removing_Malware

When all else fails, get guided help. Choose one of the specialty forums
listed at the first link. Register and read its posting FAQ. You will
generally be asked to:

1. Download and execute HiJack This! (HJT) -
http://www.trendsecure.com/portal/en-US/threat_analytics/HJTInstall.exe

2. Disable Notepad's word wrap - In Notepad.exe;  Format --> uncheck; "Word
wrap"

3. Download/run Deckard's System Scanner -
http://www.techsupportforum.com/sectools/Deckard/dss.exe

4. Save the scan results (Main.txt and Extra.txt)

5. And then post the contents of Main.txt and Extra.txt in your post at the
forum you chose. DO NOT POST LOGS IN THE MS NEWSGROUPS.

Not all tools used will work in Vista and you will need to run them
elevated. If you are unable to remove the infection by following the
general steps, register at one of the HijackThis forums as suggested.

Standard disclaimer: I can't see and test your computer myself, so these are
just suggestions based on many years of being a professional computer tech;
suggestions based on what you've written. You should not take my
suggestions as a definitive diagnosis. If you can't do the work yourself
(and there is no shame in admitting this isn't your cup of tea), take the
machine to a professional computer repair shop (not your local equivalent
of BigComputerStore/GeekSquad). Please be aware that not all local shops
are skilled at removing malware and even if they are, your computer may be
so infested that Windows will need to be clean-installed. If possible, have
all your data backed up before you take the machine into a shop.

Malke