What browser are you using?
Have you installed anything recently?
(I ask that because different software updates sometimes come with add-on {junk} you may not want, so for example a browser might also offer to download an anti-virus application, or an anti-virus app might make the offer to provide 'safe searching' and so on {and if you accept the 'junk' extras, you may find settings you previously had are overwritten in favour of their replacements}.
It used to be quite common for people to add 'free' items like a screen saver or add-ons for e-mail and then find there was a search facility {possibly called MyWebSearch} which they did not recognise but was part of the small print in the terms of the download they had made.
Worst thing was that some of these add-ons are for MS Internet Explorer and as far as I know, few can be deleted. Some might be disabled, but there are often no 'delete' methods shown for them. Also, as they are BHOs (Browser Helper Objects) they are loaded as soon as you use MS IE and some have in the past been found to include malware but as BHOs they cannot be so easily removed and were given access to data held by the browser itself.
Things may be changing wrt MS IE, but as I have Windows XP, not Vista or Win 7, I cannot switch to MS IE v9 browser, not that I really want to use Microsoft's browser, but it can be handy to know it works on websites I build :)