I've seen a trojan integrate itself so that windows thought it was missing required files whenever it tried to run an exe. If Norton mis-IDed the virus, there's a chance it didn't get the whole thing, or didn't fix all the damage. Trojans are notoriously hard to detect, most virus scanners won't pick up a trojan at all when it's attached to a file, and they're hard to detect and remove even after they install.
Sometimes Norton will put out specified scan and removal tools for a virus or trojan, and they should have manual removal instructions. If you can't find anything on Norton, you can try punching the name of the trojans into Google.
Norton has probably disabled any payload associated with the viruses, so the rest should just be repair. You'll probably have to follow the manual removal/repair options and hope that fixes it. The process will probably have you editing the registry, so I'd recommend a full backup before proceeding with it. Good luck.
There's also a chance this wasn't caused by the viruses, and is just a corruption of windows that occurred some other way. If this is the case, I'm not sure how to repair it. I usually end up re-installing windows every year; the OS slowly gets corrupted both by my trial and error programming, editing of the registry, etc. and by general use.
