[JamesC70]

There's a "name suggestion" Javascript for the Referrer field (so that newbies get a popup of other members after they type 3 or 4 characters). This name suggestion script is also used when sending someone a PM or seaching for someone on the Members List.

However, on your registration form, it's kicking in on the username selection box ... where people type their desired username. If you look closely, there's a little green or pink box under that field just before IE kicks up the popup.... that's the username selection script trying to work.

If you've added some add-on that causes this to kick in on the username suggestion field, disable it.

If you haven't, flip the source of that script off your own server and onto Google or Yahoo (AdminCP -> vBulletin Options -> Server Settings and Optimization -> Use Remote YUI) to see if it makes a difference.

Alternately, re-download the vB 3.8.6PL1 file from the vBulletin Members Area, and copy the file clientscript/vbulletin_ajax_namesugg.js from that new download to the same place on your server. Yours might have a minor corruption that I don't see from this end.