My thoughts are:
Fault finding is a logical science - providing that you don't change more than one variable at a time!
If you pull the ignition, the fuel supply and the fuel injection system simultaneously apart, then you are asking for confusion!
Firstly, in the short term abandon AS. Manually snipe if you have to.
Change your password.
Is your password stable? Does it last?
If the answer is no, then the problem lies elsewhere.
If the answer is`yes, then after a month (no less, in case there are leads & lags elsewhere in the system) rejoin.
Firstly I would assume that you have a Trojan. Despite the faith you have in your anti-spyware products, I'm not as confident as you. The two you mention I have used and used to trust implicitly because they went about their work so impressively! What worried me was that they then found things the other had missed! So with efficiency at say 80% can you trust them?
I am now using Microsoft anti-spyware beta and, to tell the truth have not gone back to either Pest Patrol or Ad Aware. It is better at detecting intruders, certainly, than the other two. (not 100% I might add and that's why I have another anti virus program) I use Avast which seems pretty good at detecting things (the database updates automatically - or so it tells me - evrry couple of days.) It is also freeware (& not crippled either!)
Now so far so good - two programs alerted me simultaneously (a good sign) that a nasty was trying to download! Where they could not agree was where the originator was. I spent hours running this, then that, putting this & that into quarantine.
I then had a thought - I clicked on a tiny, tiny software programme called "Startup" by Mike Lin www.mlin.net. Bingo - discovered in one - a new .exe had added itself to my list in HKLM/run. I then could disconnect it from the startup directory and remove it manually the next time I started the computer.
So why didn't these "sophisticated" programs spot it? Not a clue. However you must know what is in your HKLM/run to start with - don't wipe out all the exes you might stop an essential function (like your mouse!) This little free program lists all the essential startup registry entries and all you do to stop something running is untick a box - much, much easier than ferreting in the registry with all the attendant risks that involves!
Good hunting. Remember nothing I have suggested will cost you money - only time!