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.

I want to know what the difference is between V3 Internet Security 2007 Platinum and V3 Internet Security 2007 Platinum for 3 Users (Family). I dont care about the virus protection. I just want to know the difference. Can I use the single one for one computer. How does the Family one work? Does it work for different users or like accounts?

there are so many different registry clean ups being offered and they're SOOO expensive. I'd like to find a good free one that is actually free…not a trial that lasts thirty days and doesn't actually do anything. Any suggestions or advice?

I have Norton anti-virus and spyware.