Well, if you've tried disabling all of the start-up items (in CCleaner or msconfig) all I can say is clean install. You don't remember doing anything, changing anything, installing/uninstalling anything before this started happening?
Or try looking in HKEY_CURRENT_USER>Software>Microsoft>Windows>CurrentVersion>Run
See if anything in there looks like a likely candidate. That's the best I've got for now.
For whatever reason... i kept going into "Services" instead of Startup as if it's a conditioned response or something... Sorry about that. I've found one of the problems and disabled. I'll have to exterminate it completely somehow. As for the STEAMA~1.COM item, i have no idea how i'm going to find that, now...
Checked both registry pathways. Some Windows Media Centre and Nvidia items. As well as some items without set values or names. Just displaying as "(Default)".