Avast just popped up claming the hosts file and nv4_mini.sys are “Rootkit” because they are hidden files and it wants to delete them.
I think not! They’re the system’s hosts file and part of the nVidia video drivers.
I’m running XP Pro SP3, Avast 6. Everything up to date.
It would really help if you updated your signature with your system specifications.
In Account Related Settings select Hide email address from public to prevent scammers and spammers harvesting your yahoo.com email address.
I only have seven systems around here. 
Three laptops, two desktops, a Cybernet Elite II and a G3 iMac. Plus a third desktop waiting to be put together.
How many pages of system specs do you want?
The issue at hand is why Avast would suddenly decide the hosts file is part of a rootkit? Same for one of the critical files for the video driver? Deleting either would cause problems.
Avast has also recently tried to delete, several times, update files that Microsoft Malicious Software Removal Tool has downloaded. I keep telling Avast to ignore, it keeps wanting to delete.
I very much doubt MSRT is malware, though it and MSE refuse to acknowledge the GameVance remover is NOT GameVance and constantly want to delete it from my folder of cleanup utilities I use for computer service work.