slowly web with XP and linux

hi,

I use a laptop with a “freebox” to connect to internet (DHCP) ethernet

First, with linux on HD : no problem : speed 1-3 Mo

A few weeks after, with XP (no firewall, no antivirus):no problem

A few days after : with linux : no speed : 5 ko (on usb , on ethernet, worth)

Same thing with XP, linux on HD and linux live CD.

With anither computer : no problem.

Is it harware ???(laptop buied in october )
Is there a virus on boot ? But avast does’nt see it.

thanks for help and soory for my bad english