Okay, posting this for future reference of other ppl.
I called Mac Support again (they just love me now sarcasm).
First guy I got was an idiot who asked why I was even scanning my system, since Macs don’t get viruses (because those viruses in the Avast! definitions, I mean, they don’t exist, am I right? :P).
So I lied and told him I sometimes run windows files to shut him up. Then he asks if I’m booted in windows and frustrated I say look, I’m just scanning with Avast! in Mac Mode and I’ve got an error on this file called “bootroot.loader.dmg”, it’s a decompression error and I want to know if this file is native to your OS or if it’s nasty and I need to dump it.
He says, “I don’t know what you’re talking about.” I say, okay, do you have tiered tech support? He says, “What?” I’m like tiered tech support, like can you elevate me to a higher level of tech? He says , “Um yeah, but I need to try to figure it out first.”
Ummm…wtf? He just admitted he had no idea what I was talking about. So this time I explain it to him again slowly, and he puts me on hold. He picks up again and asks what the problem was again. more frustration I tell him once again that I have a decompression error, Avast can’t scan because it’s taking too long and I want to know if it is a natural file in the system.
He finally gives the heck up and transfers me to a higher level tech support, who asks me what the problem is. I tell him, and let him know the address of it in the system. He types into his computer and says “Yep, that’s native to the OS. Don’t delete it.”
I had a lovely chat with this tech guy, he basically said in cases of Decompression error I should do exactly what I did, which is to figure out what the file came from (to determine if it’s bad or not).
So yeah, if anyone else needs to know, mediakit.framework is part of the OS (Mac OS X 10.5.8), and so is bootroot.loader.dmg. They are supposed to be there, and even if something did try to get in, so long as you are running a standard account like I am, it would have to ask you anyway.