I heard that if you download an antivirus program and another antivirus program while the old anitvirus program is still installed, your computer would be all messed up and glitchy. Right Now I have Avira, but Avira doesn't fix the RUNDLL Error, and I look at yahoo answer, nothing work, and youtube too.

So if I download a Registry Cleaner and installed it while Avira is installed, would my computer be messed up? What happen if I download a spyware, malware, registry cleaner, and an antivirus program? What would happen with my computer with all these programs? Since Avira doesn't protect spyware, malware, and doesn't do a registry cleaner.
What would happen if I download with anything in any particular order.