In reply to Pete Ford:
OP - Check the chrome extensions and delete which ever you haven't purposefully downloaded (they've usually got really obvious names). If it won't let you delete it (can't remember the term it uses but looks like an admin reason) note the reference code then go to
C:Users<yourusername>AppDataLocalGoogleChromeUser DataDefaultExtensions
and delete the one that matches the reference code. (if none do try matching extensions that you do want against the codes in the folder and you'll likely find one that doesn't fit them, delete that).
This is REALLY common on Chrome recently though they're finally taking some steps to fix it. Note that running 113 different anti malware programs as many will advise is utterly unnecessary if it's a dodgy extension. If it's not that then there's other steps you can look at, including the one I'm about to write out for Coel.
Coel - If you use Chrome you could try the above first. If not, or if that doesn't fix it you need to look at the list of your installed programs and find the one that looks dodgy (they're normally obvious). If you've got quite a few you don't trust then you might need to google some to work out if they're required or not. This might differ in different versions of windows but you should be able to find the list under Control Panel and something like
Programs and Features.
Both - you might find after removing the offending software that when you restart your machine it reinstalls. This will be because it's put something in your startup list to do so. Go to Start-> Run and type msconfig.exe. The window that pops up, click on the startup tab. Find the dodgy one and deselect it.
It's also worth looking at other things here to see if you really need them to start immediately and automatically when you switch on. Lots of programs will add themselves to the startup which don't need to be there and this can REALLY slow down your startup. (I've just noticed Spotfiy had got itself back on there after an upgrade a while ago so have removed it - I'm happy for it to start when I ask it to)