I'm running a gentoo distrib of linux.
In my network, we have a brother MFC 3420C multi function printer
which is connected to the usb port of a computer running Windows XP, i
would like
to be able to print from my gentoo box via the windows Xp station ,
what should i do ?

Use CUPS (http://www.cups.org) and Samba (http://www.samba.org).
This is: add the remote windoze share printer to the local cups
server. Something like this:
smb://user:pass@workgroup/server/sharename

i have tried this won't work since i have to use a driver on my linux machine.
Linuxprinting.org don't make references to drivers usable by that printer.

Their must be 10 ways to do that.

But how about this idea;  run a web server on the Xp machine.
Have  a form that works with standard web browsers that accepts files
to upload for printing.

The Xp machine web server then prints those files, your form can have options like 
how many copies.  Or what ever else you would like.

yes that's should work,
but i never installed a web server, so i believe it will be too long
for me to do that. thanks however :)

have you tried this?:

http://www.brother.com/cgi-bin/agreement/agreement.cgi?dlfile=http://solutions.brother.com/Library/sol/printer/linux/rpmfiles/lpr_others/MFC3420Clpr-1.0.0-1.i386.rpm&lang=English_lpr

(you would still have to use cups or samba somehow with this driver,
so it's not trivial)

yes i have tried it without luck since i'm running gentoo
thanks however

try the following:

edit /etc/make.conf
append to USE= cups and samba

re-emerge samba and cups
# emerge samba cups

add your samba printer to /etc/cups/printers.conf
<Printer samba>
Info lpwa2
DeviceURI smb://user:password@domain/server/printername
State Idle
Accepting Yes
JobSheets none none
QuotaPeriod 0
PageLimit 0
KLimit 0
</Printer>

# /etc/init.d/cups restart

and it should work

if you want drivers, don't forget to emerge foomatic and enable the ppd USE flag.