Hi
Pagefile.sys is a fundamental system file. It is not of itself a virus.
It is supposed to be a virtual memory overflow when the O.S. runs out of RAM, so I would expect any decent anti-virus to detect a virus BEFORE it gets flushed from RAM to Pagefile.sys.
XP is quite selfish, so it does not allow users to open Pagefile.sys.
Either AVIRA has privileged access to Pagefile.sys when properly installed, and something went wrong for you, or they ought to know better than to try !!!
I am sure that expert assistance will come by soon - otherwise you could try AVIRA support.
Regards
Alan