Security status looks good...so what? I suggest you to buy antivirus software. Personally, I don't trust these free thingies. I'm paying for a good program and never experienced any problems. The only thing I get to see about viruses is the program's message "virus warning" and "action blocked"
If kernel32.dll, user32.dll and shell32.dll (there are simply no more important dll's ever in winblows) are modified, as AVG says, then the Winblows should replace the files with original files. It is called WFP (Winblows File Protection) and if you trying new, dubious software, you should not modify win the way that disable WFP. I have WFP disabled and removed, because I like to using my modified DLLs, but I know what I doing and I never try new, dubious software. What about to replace these few files from the Vista org. DVD? That should fix things. Just rename the file - eg. shell32.dll to shell32xxx.dll and copy the new one in the same directory with proper name. Repeat for all the other files and reboot. Then search for "xxx" in the file names and delete the modified (?) files. End of worry/story.