Vlan Problems with software

Hello i have a catalyst 2950 switch with 9 vlans and 1 trunk that is
directly connected to a linux box for routing the linux box has other 2
interfaces, im having problems with all types of software that have undergo
routing with the linux router in the middle. 

This are the simptoms:

Lack of connection and unable to authenticate to a domain server, cant use
RDP client. But in all of this chaos i found that one computer was able to
do everithing with out any problems.

So looking at the inteface of the win xp i noticed that it was a 3com card
and had enabled 802.1p, i disable it and it had exactly the same problems as
the rest, I enabled it again but unfotunately not all network card have this
available.

Is there a way to overcome this issue either with the linux box or using the
catalyst switch

Thanks for your help.

Fernando Rodriguez
AITelecom