You need a product that specifically looks for Trojans. Trojan Hunter has a free trial that you can download click here If that doesn't get it then you may need to get some help to root out the trojan manually.
Anti-viruses are what they say: "against viruses", so don't necessarily deal with trojans, tho they may find them. Download and install all that Rdave13 recommended (but of course not 2 antiviruses at same time, and you already have Tesco's). Update them - if offered that option - and Run them. Nellie2's advice is correct - you probably need an "anti-trojan", so use her link too.
If you've still got the problem after following the advice above, have a look at click here This will tell you about sillydownloader and contains a link to the Microsoft Knowledgebase that tells you how to do a registry edit to get rid of it. Don't edit your registry unless you know what you're doing!