I have had this problem for a while now and I thought I had erradicated it, but alas, no.
iexplore.exe opens by itself and seems to be surreptitiously downloading a zip/rar file named svchost, then extracts itself.
I think I've now got a handle on the file names and I've renamed iexplore.exe to iexplorer.exe to try and stop it from happening before I solve the problem. Please help !!!
I use avg free antivirus, Spybot, webroot spy sweeper and zone alarm firewall.
In this case, I'd recommend a full scan. If that doesn't work, prepare to format your drive.
Since you don't know the extent to which the virus(es) have permeated your drive or from what date for that matter I wouldn't recommend using any of the backup disks that you've made.