Note that many websites have their own advertising, unrelated to adware.
ShopNav is a hijacker or it's distributed as an Internet Explorer Browser Helper Object. This program also launches an updater process run at startup to ensure all files are present on the infected computer.
Address bar searches, the Search explorer bar, unknown domains, and, in some variants, non-www server names entered into the address bar without the preceding 'http://' will be sent to Srng's controlling server www.srng.net, which redirects to a search service at apps.webservicehost.com.
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When installed it sends details including your Windows account name and your previous search settings to its controlling server.
It appears that Shopnav have taken their products down from there site. When visited it reads this:
Thank you for your interest in our search enhancing products. Unfortunately, these products are no longer available.
If you wish to uninstall, please click here.
They now supply a remover program on their site:
Manually: Open a DOS command prompt window (from Start->Programs->Accessories) and enter the following commands, for the IE variant:
regsvr32 /u "\Program Files\Srng\SearchHook.dll"
regsvr32 /u "\Program Files\Srng\IEHelper.dll"
Or for the SN variant:
regsvr32 /u "\Program Files\Srng\SNHelper.dll"
Removal instructions from the vendor.