In searching your problem there seems to be quite a few recent posts with the 0020.dll error, it is probably a new virus or rootkit.
It sounds like you are getting close to the point where you will have to reload the operating system (or you can wait, with your fingers crossed, and hope that the anti-virus makers come up with a fix).
I recommend that you start asking everyone you know if they have the same computer that you have (or one of the same brand with a slightly different model number) and if so do they have a system disk that you could borrow to use with your key (the key not the CD or DVD is unique to your computer, the key also relates to the software disk so disks form other computer makers do not always work (there are ways around this but they are piracy or very close to it). Microsoft tracks key usage and if too many computers show up with the same key they blacklist it).
Today, before proceeding, download a keyfinder and run it then make a permanent record of your operating system key. If you cannot retrieve your key it will do you no good to get the OEM operating system disk, you need your key to use it.
If you can retrieve your key, call up the customer service for your computer and ask them for a copy of the operating system on a disk so that you can restore it, or at least how much it would cost. The price will enable you to make a decision on whether to get a replacement operating system with license & key or to go with the OEM bundle.
I would also start backing up the important files on the computer to Cd's or DVDs so that you loose as little as possible (use DVDs if you have a DVD burner, they hold more per disk).
If you know a techie ask if they can sweep your computer, for malware and virii (I regularly do this for local friends).
Enjoy!