This problem happned to me in the past and I kept notes on how to fix it but lost them...
The problem happnes like once a year maybe twice:
Xbox starts running slow, when changes screens or navigate through the buttons media takes longer to load
but in majority runs the media fine.
it then starts not booting sometimes and gets worse and worse in a few days. some times it booted in a thing called slayer or just the normal xbox screen (im used for avalanche to load and me clicking on media centre)
it takes for ever to load and I have to reset it like 30 times before I might get avalanche to prompt
I have never upgrded the dashboard in the past and only upgrade the xbmc t3ch version everytime this happnes. (cause it runs fine)
I remmber that I had to delete some folders but also some files like the thumbnails etc...to fix this issue Im not sure what other folders I need to remove. I did this but still no good. I even put a whole new userdata folder and loaded it up but still slow...si I went back to my own.
I dont think its the HD cause this happnes all the time...please help me out!