I'm not sure what you mean by the phrase 'attached to this particular file' (is 'Bs86vrnx.default' a folder in which firefox stores cookies - anyone?) but if all that your AVG is finding are tracking cookies, that's nothing to be concerned about, in itself. Cookies are harmless text files. You will pick them up repeatedly (unless you set your browser to reject them), because every time you visit certain websites, those cookies are planted, yet again, on your computer, even if you've previously removed them.
You can't do any harm by deleting them, and they can't do any harm to you. (They're often seen as a threat to privacy, but they're so easy to control that it's not worth all the fuss that's made about them.) There's a guide to cookies here:http://www.bleepingcomputer.com/forums/t/45333/cookies-a-complete-guide/
I don't use Firefox much myself, but it seems quite straightforward to restrict what cookies are planted on your computer. Click Tools, then options, then privacy. In the 'Keep until' box, choose 'ask me every time', and you'll then be warned every time a cookie is offered, and be given the option to reject it. Others may offer some more convenient methods. Otherwise, just keep deleting them when your antispyware programs find them.
This said, I can't find any such folder 'Bs86vrnx.default' on my computer, though I do have Firefox installed, so I'd be glad if anyone can clarify this reference. (Firefox cookies seem to be cached in 'vu6nbj6.default' on my machine.)
Edited by Alan D, 21 March 2008 - 12:31 PM.
Windows XP Home SP2; AVG 7.5 Internet Security Suite (AV/AS r.t.p, and firewall); Windows Defender (r.t.p on); SuperAntispyware Free; a-squared Free 188.8.131.52; Spybot 1.4 (Immunised, but no Tea-timer); AdAware SE Free; AVG Anti-Rootkit Free; Spywareblaster; MVPS Hosts file (with HostsMan); McAfee Site Advisor.