... and nothing was found, yet you have problems booting up, chances are very high that one of the viruses, or removal of such, has damaged critical system files.
By the way, did you run all of these sweeps WITHOUT the internet connected and with your system restore disabled? If not, you may STILL be infected. Did any of the sweeps reveal anything found at all?
You can try a repair install of Windows over the top of the existing install which will, hopefully, retain all of your data even though you'll need to reinstall all software. Be sure to backup your data before you try it and be sure to unplug any external devices (printers, HDD's, etc...) while it's being brought back up to speed. Once you have antivirus and all of the Windows service packs and critical patches installed, you can begin reinstalling the rest of your software.
But, once a system is as damaged as the one you're describing, you're going to be better off in the long run if you just save your data, wipe the drive and reinstall a fresh copy on clean drive, then install all drivers for what's in the system, then getting antivirus installed and going online to get your service packs and critical patches. Don't go online till you have your antivirus installed. Again, once that's done, start installing your other software and external devices.
