I also suspected something in that direction. Not that I'm an expert, it's more a matter of general experience and gut-feeling.
Like I said, when I bought this Acer Aspire One netbook, I de-installed the pre-installed anti-virus software (probably McAfee) through the regular XP procedure (in order to replace it through Avast), but this probably means that some files and entries may still be around and cause damage under certain conditions, if I've comprehended well.
How do I find them? Under C:\Programs, there is no folder carrying a name with any resemblance to this previous software. There was one under 'users', and with contents, but deleting it didn't solve the problem. Searching for 'mcafee' produced no results. How would I totally get rid of all the 'hidden' McAfee files, entries etc? Would McAfee inform me? I doubt it.
PS
Could you otherwise send me the installation file of the previous version to my email address registered by you, please? I didn't save it.